- Article/photo's courtesy of:
- Eric Hummel, AKA Grim-Reaper
AZ-Kickin Rear Disk Brake Conversion
I deal with a lot of Mud on the trails and drum brakes HATE mud. Just about every trail ride I was having to pull the rear brakes apart and clean all the dirt out of them and manually adjust the brakes to get any peddle. I was over it. Disc brakes generally will clean themselves out on their own in just a few feet of travel. They self adjust with the fluid so no jammed adjusters to contend with. I decided it was time to do the swap over to put an end to a time consuming ritual.
The Disc conversion kit I used was from AZ-Kickin. I used their recently release 10/12 bolt conversion kit. The kit comes with a healthy set of CNC machined caliper mounting brackets, Stainless steel spacers, all the hardware to mount them including new wheel studs. This kit also includes a new set of Quality rotors that have been machined to work with the factory axle shafts. All in all a very complete set up that saves a lot of time over buying just brackets and having to take a set of front rotors to a machine shop to get the centers turned to work. When you do the math and add up the cost of rotors locally, cost to have them machined to fit the axle flange and the time it will take to get all this done, it is a VERY competitive price for the kit. The only thing you have to supply to complete the axle end of this upgrade is the calipers you want to use and a hose to connect them up.
Az-Kickin disc kit for 10/12 bolt
76-78 Elderado calipers with parking brake
The Az-Kickin brackets are sized to fit Stock GM truck front calipers and a few other calipers used on GM vehicles. In 76-78 Cadillac Elderado's ran 4 wheel disc and the rear calipers have a parking brake provision. These will work with the Az-Kickin brackets. After looking at the options I thought the Elderado calipers would be the best choice so I could retain a mechanical parking brake. The other option would be to use a line lock or making some other sort of parking brake. All the other options for some sort of parking brake have possible drawbacks that would make them useless in an emergency. I hunted around for the calipers and the best price I found was at the local Advanced Auto parts.
You have to remove the axle shafts for this project. To get stared you need to get the rear axle up on a set of jack stands. Remove the wheels and drums. Drain and remove the differential cover. All carriers that I am aware of for these two axles have a similar set up and require you to remove the center shaft that is retained by a locking bolt. Once the shaft is out of the way you can push in on the axle shafts about 3/8s of an inch. This will release the "C-Clip" that retains the axle shaft. Fish the C-clip out of the differential housing (a magnet is the easy way). If you are dealing with an open or Gov-Lock differential be careful not to spin the axle shaft. It's possible to cause the side gears to fall out if the axle shaft is turned while the cross shaft is out. With the C-Clips removed you can now remove the axle shafts (all of this is covered in your shop manual and is pretty straight forward).
The drum backing plates are retained with four bolts. Take the bolts loose and disconnect the parking brake cable and brake hard line (cap the line so the Master cylinder does not run empty). You will not need any of these parts. Inspect the axle seal and replace if it's questionable. Stuff a clean paper towel into the end of the axle to prevent any dirt from getting into the bearings. Clean the mounting flange and make sure there is no Dirt or rust that would cause any problems with alignment on the Caliper mounting bracket.
After removing drum assembly clean mounting flange and inspect seal and bearing
Mounted Az-Kickin caliper bracket
The Az-Kickin brackets clock the caliper towards the top. This gets it up out of harms way from rocks and things you may come across on the trail If your running a shallow back space rim this is a big plus. The bracket bolts on with three Grade 8 bolts on the inboard side of the mounting flange and will position the caliper at about 1-2 o-clock.
To install the Disc rotor you will need to hammer or press out the old studs. The new studs are stock front wheel studs and need to have the holes on the axle flange enlarged to 17/32's. This is not a standard size drill bit you are going to find in your local home store. I was able to locate an Industrial Supply place in my Yellow pages called Fastenal. They ordered me a high quality bit for about $20. If your shocked at that price then you have been buying cheap drill bits. Once you use a quality bit you will understand the difference between a good bit and a cheap one.
Drive out old wheel studs
Drill out holes to 17/32's
You need to be very careful to center the bit on the existing hole as well as having the drill perpendicular to the axle flange. It can be done by hand but if you have access to a drill press it would be better. If your off on either the drill angle or center of the hole then the lug nut is not going to be centered and you will have a problem with the taper seat on the lug nuts not centering on the wheel. Be sure to use cutting oil to lubricate the bit. Drilling steel dry, no matter how high the guilty of the bit, will result in a dull drill bit in just a couple of holes. In a pinch motor oil will work. Another mistake that people often make is running the drill at too high of speed. 500 or so RPM is plenty with a slight amount of pressure. Too fast will cause the metal to "Tool Harden" and become more difficult to drill through. It will also cause problems with the temper of the drill bit.
After you have enlarged the holes, make sure there are no burs in the holes or on their lip. It would also be a good idea to sand the mating surface clean of rust and grime. If something didn't let the disc sit absolutely flush to the flange on the axle it can cause a number of problems. Some of this would include a pulsing brake peddle, similar to what a warped rotor will cause. The worst thing that could happen is whatever didn't let the disc seat would crush or come out and cause a loss of torque on the lug nuts that could lead to a wheel coming off. So don't get in a hurry on this step. I used a cheap disc sander attachment for a drill with 180 grit paper and it only took about one minute to get down to where you could see the original machine marks on the flange.
Installing the wheel studs is next. The disc is installed from the backside of the axle flange. The wheel studs pass though it. The proper way to install a wheel stud is with a shop press that can exert about 2-3 tons of pressure to make sure they are fully seated. Again the danger of not getting the studs seated could ultimately end with the loss of a wheel. It's worth the time and a few bucks to take this to a machine shop to have them pressed in if you don't have access to a press. If you just don't have anyway of pressing them then the next best method would be to drive them in with a Drift and a hammer. Regardless it would be wise to keep a lug wrench handy for the next couple days and check the torque on the lug nuts when you stop. The heat cycling of the disc and flange, from braking will, cause any studs that are not fully seated to loose torque so play it safe and check to make sure they are still tight over the next couple days.
Clean mounting flange to remove any rust that may not allow rotor to seat flush
Press in new wheel studs
Now a rant.
It is absolutely the wrong way to install wheel studs by drawing them in with a lug nut PERIOD! Wheel studs are normally only torqued to between 80-120 ftlb on the average passenger vehicle. To create enough force to draw a wheel stud in your going to need to produce well over 150 ftlb of Torque, probably over 200ftlb. That's well in excess of what that stud was designed to handle. You are weakening the stud if you do this and there is a VERY real danger that they could prematurely fail and cause the loss of a wheel. If some monkey mechanic ever tells you this is ok to do and they are about to pull this stunt on your vehicle LEAVE. I can about guarantee that at least 50% of the cars you see sitting on the side of the road with a missing wheel is because of some knucklehead pulling this stunt.
End of rant
Installing the caliper is no different then the front. I always use a anti squeal coating on the backs of the brake pads. This stuff seems to do the trick and I have never had noisy disc brakes. Be sure to lubricate the slides and pins with a high temperature grease or anti seize so they can slide freely. Most parts stores sell a product that is made for this application. You don't need to tighten every thing down yet because you will need to pull the calipers loose to bleed the brakes. Having the caliper loosely in place will help with routing your brake hose.
With the Caddie Calipers I did some searching and found that a front brake hose from a 79 Chevette worked out great. I had to do just a little bit of grinding on the banjo fitting end to have the hose pointed in the direction I wanted but it was otherwise a great choice. I am pretty sure this will also work on the Stock front calipers if you decide to use them. The other end of the hose should go to a bracket to hold it. You don't want to let the hard line in the brake system be the support. The line will eventually fatigue and crack from vibration if it's not properly supported. I made a simple L bracket from some sheet metal. Drilled a hole and spent a few minutes with a Dremel to get the octagon shape required to properly fit the hose so I could get the locks on. This hose has the correct 3/16 fitting that matches the one on the hard line. You just need to carefully bend the hard line to connect to where you mount the end of the hose. If you don't have a proper tube bender you can use a 1 inch diameter socket to help to form the bend. Just be very careful not to kink the line.
Fabricated hose bracket and new hose locks on 79 Chevette hose
Final test fitting before bleeding lines
Well that's the easy part! It all goes down hill from here!
These trucks never came from the factory with four wheel Disc brakes. There is going to be problems with the Front/Rear braking bias. If you just hook up the rear lines and bleed the system out your going to have the rear tires locking all the time. This is a dangerous situation.
I did a lot of research on a solution to the problems I was facing. I was able to obtain a LOT of information from our ColoradoK5 members and their experiences. This helped me plan a course of action to work out a way to adjust the braking bias. I decided to take baby steps and see what each change did to bias, feel as well as overall stopping power. My goal was to find what modification gave consistent results every time so that if you the reader take on this conversion will have the same results.
Under ideal circumstances, the best way for the brakes to be biased is that the fronts lock just before the rears. If the rears lock first then there is the very real possibility that the rear of the vehicle would come around and put you into an uncontrollable spin. If the front brakes lock then the vehicle will continue to travel in a straight line and this is a MUCH more safe thing to happen. Normal brake bias is between 60-70% biased to the front of the vehicle. A lot of the bias depends on the weight on each axle, wheel base, even the spring rates on the suspension as well as the vehicles Center of gravity. A short wheel base vehicle tends to have more weight transfer to the front on a hard stop so it will have more front bias.
Brake bias is controlled by many different things. The Combination Valve (sometimes referred to as a Proportioning valve) is a big part of it. Since these trucks never came with four wheel disc, there is not an easy solution of swapping the Combination Valve (CV) to a disc/disc version of this truck. There is a possibility of a disc/disc CV is available from inline tube. Problem with this is I don't know if the bias is correct for these trucks but they do offer a CV with the correct fittings if you want to give it a try.
In my search for the right way to solve this problem I was getting a lot of different information on what worked. Several people said they had good results by leaving the stock CV in the system. Several said they had to remove the CV to resolve problems. Most said they used and adjustable Proportioning Valve (PV) in the rear circuit to control the bias. Some said they just hooked it up and were pleased with the results and did no other modifications. I had no clear answers so I tracked what changes were used the most and decided to follow the highest average. I am still trying to come up with "why" some people had certain result that others didn't. All I can come up with is the age and wear on these older vehicles. Some of these vehicles may have defective CV's is the best explanation I can come up with. With that theory in mind then almost everybody would have the same results.
The first obvious item I needed was an adjustable Proportioning valve. These are easier to find than I thought. Online you can get these at a number of places including Jeg's, Summit and In Line Tube. They run about $50 plus shipping. I hit the online Yellow pages and searched under "Race shops" and found a place less than a mile from where I work and he was able to have me one and have it the next morning.
I ended up using one manufactured by Wilwood. This adjustable proportioning valve was capable of removing 40% of the pressure going through it. Now that may seem like a lot but the fact of the matter is using the front calipers and rotors on the rear you have FAR more braking capability then you need. If you ever mess with a car that came from the factory with rear disc, you will notice the rear rotors and calipers are smaller then the front. This size difference also helps control the bias. The piston diameter has a HUGE effect on braking potential even with the same line pressure. This is why the bias becomes so far out whack when you use a front disc and caliper on the rear.
Wilwood Adjustable Proportioning Valve in rear circuit used to control brake bias
Installing the Wilwood PV was not difficult but it did require a few adapters to match up to the master cylinder (MC). GM brake systems use a lot of non standard fittings. This presented a lot of problems along the way. The rear circuit on my 75 used ¼ inch tube. The PV used 3/16. I was lucky and my local parts store had an adapter for the Master cylinder end. The return line I ended up having to use a couple adapters as well as cut off the end of the line and replace the flare nut with a standard size. The flare fittings used in brake systems are known as a "Double Flare". Not hard to make but does require a flare tool with the double flare nipples. Again my local parts store had this in stock.
Where you put your PV is up to you. It will be fine under the hood and you would just need a couple short lengths of 3/16 tube to connect it up. I decided to be fancy and run mine into the truck so I could adjust it when I was in the seat. Mine is mounded to the bottom edge of the dash. With a little bending and tweaking of the tube it worked out quite well. I needed two 40inch long 3/16 tubes to get it to that location.
When it comes time to bleed the system you will need to take the rear calipers loose and put them over the disc below the mounting bracket to get them to bleed properly. While locating the calipers up out of harms way is a good thing it positions the bleeder screws at an angle and you will not get the air out if you try to bleed them installed. You must have the bleeder pointing straight up. Tapping on the caliper with a wrench will help dislodge and air bubbles as you bleed the air out.
For the first round of testing I left the factory CV in. I bleed out the system and drove it around for a little while playing with the bias on the PV. The brakes worked and stopped the truck fine but the peddle travel was excessive and I didn't like it. I kept messing with the bias and I was getting inconsistent results.
During my research and talking with other CK5 members, I learned a lot about the way the CV in our trucks work. In an information post I had in the forums, several of us debated what to do with the stock CV. Like I mentioned before, many people had left the CV alone and were happy. I wasn't, here is what we came up with why I had the peddle travel. The stock CV is three parts internally. On the front circuit there is a metering valve. Its function is to hold off applying the front brakes till the rear brakes build pressure. It doesn't activate the front till there is between 70 and 120 PSI of pressure (Discs require between 900 and 1200psi). In the middle of the CV is a Pressure balance switch. It's sole function is to activate a light on the dash if either brake circuit looses pressure. In the rear circuit there is a proportioning valve that controls the overall pressure sent in stock form to the drum brakes.
Combination valve disassembled. Front metering valve on left, Proportion valve for rear on right
After a bit of online talking with others doing this type of conversion we pretty well decided that the stock CV was causing us problems. My thought is with the lower pressure coming into the rear circuit, due to the Adjustable PV I had installed, was causing the stock PV to act erratically. It had to go. I again ran into problems with the odd sized fittings. I found the parts needed and again to bypass I was going to need to cut the ends off and reflair two of the lines. Well I was going to have to remove several inches of line to get to a point where I could have a straight piece of tube that was suitable to flair. I was probably going to not have enough tube left to make the connections without rerouting or replacing some tube coming down from the Master. I decided to see if I could disable the stock CV and basically make it into a distribution block. This would avoid having to cut off the existing fittings and save me some trouble.
Once I removed the CV I studied the diagram trying to figure out how to disable the unit. The proportioning valve end was easy. Simply unscrew the end and it's apart. I studied how this thing worked and compared it to the diagram. I found the best way to disable was to snap off the part in the diagram marked "Valve Stem". This piece has the actual seal on it that controls the flow on the rear circuit. You just need to grab the end of it and bend it back and forth with a pair of needle nose pliers a few times and it will snap off. Clean out any crud in the bore. A thin coat of brake fluid or lithium grease on the seals to ease reassembly and that end is done.
Understanding what the metering valve did I decided it was no longer needed for a Disc/Disc system. I already had too much rear brake. The metering valve was holding off about 100psi on the front circuit. The rear circuit was uninterrupted flow and it made sense to make the front the same way to get them activating as soon as possible. This is a little more tricky to get apart. Remove the rubber boot on the end to start with. If you look at the end you will notice there is a pin in the center and a snap ring retains the assembly. To get the snap ring out you have to push the nub in without pushing on the pin. Then you can pry the snap ring out and the metering valve assembly will come out. Disabling this was easy on mine. The seal that controls the pressure is right on the end off the assembly. It sits in a metal cup. The metal cup has four slots around the end and a spring presses against it. The spring provides the pressure to keep the assembly against the lock ring as well as to control the hold off of 100 psi but it must remain there to keep the unit sealed. You can pop the seal right out of the metal cup and this will disable the metering valve.
Valve Stem that I broke off to disable PV located in center
Seal removed to disable Metering valve
Now here is the problem that you may run into. There was a lot of deposits and rust in the bore that the metering valve sits in. When you compressed the metering valve to remove the snap ring there is the possibility that the seal ran over some of the crud in the bore and was damaged. Inspect the outer seal carefully for any tears or cuts or grooves. I got lucky and only pressed in just far enough to get the snap ring out and didn't damage the seal. Clean out the bore before reassembly and be very careful not to score the wall. Little light grease or brake fluid on the seal to ease installation. Bolt it up and hopefully it will not leak. Mine hasn't and it saved me a lot of trouble getting the fittings worked out needed to by pass.
What I did run into was some trouble getting the CV bleed out. Air wants to rise in fluid. Both inlets to the CV are on the top. The front circuit bleeds out fine since one of the lines out is also in the top. The rear doesn't and you could run a gallon of fluid through it and never get all the air out. Well when I had the CV apart I noticed the inlet for the rear was at the very top of the bore all the way in the back. If you look at the CV mounted on the truck it's angled and I think this was deliberate so that you can get the air out. All I did to get the last of the air out was loosen the rear inlet a few turns so it would leak. I tapped on the CV with a wrench to shake the bubbles out. The MC will gravity bleed when you do this so air is not going to easily go back up that line. The air will exit with the fluid that's leaking out. Worked like a charm!
I drove the truck for a few days and played with the Wilwood PV getting my balance right. Through this whole conversion the one thing that bugged me was peddle travel and a spongy feel. It was better once the CV was disabled but it was still there some. I kept driving and it kept getting better. After about 80 miles they are great. The peddled came up and became nice and firm. I can't explain why it changed. It was gradual but it did come up to a point I'm very happy with it. I was going to start looking at going to another Master cylinder from a vehicle that had four wheel disc from the factory thinking my problem was a lack of volume for the rear disc's it irritated me that much. Now the peddled travel and feel is not much different than my wife's stock 79. This may have been something goofy with just my truck. It may have been the new rear pads took a while to seat since they do much less braking than the fronts. Hard call as to what it was but even with the stock Master Cylinder I ended up with excellent results after all. I had my concerns that the Stock MC was going to be up to the task but it was.
One thing I do need to warn you about. If you look at your Master Cylinder you will notice that one reservoir is smaller than the other. Drum brakes do not use much fluid. From the word go they use about 60% of the fluid they will ever need. As Disc brakes wear, they adjust themselves with the fluid volume in the caliper. That's a big reason the front reservoir is about 4 times the capacity as the rear. It's enough capacity that the brake pads should be able to go from new to worn out and never risk the reservoir running dry. The rear reservoir does not have enough capacity to do that. You need to make sure to regularly keep an eye on the fluid level of the Master cylinder and keep it topped off.
Well I still have not had a chance to work out hooking the parking brakes up so there will be a second part coming in the future dealing with just those. I wanted to get this out because I know a lot of people are contemplating doing a similar conversion this winter. Hope it helps you out and I will add in the parking brake information once I get a chance to finish working that out.
Az-Kickin 10/12 bolt disc brake conversion kit.
77 Cadillac Elderado rear disc calipers (Left and Right)
Four caliper guide pins
79 Chevy Chevette front brake hoses.
Two Banjo Bolts
Parking brake levers (L&R) and two return springs.
Wilwood Adjustable Proportioning valve. Jegs, Summit, Inlinetube.com
Assorted fitting adapters. Local parts house
2 brake hard lines 3/16 diameter 40 inches long to mount PV to lower dash. Local parts house
17/32 drill bit Fastenal
Quart of brake fluid. Local Parts house.
I want to thank all of the fine people at CK5 for their information and relating the experiences they had when doing their disc conversion. Your help was key to figuring out what worked so that the next guy can read this write up and know exactly what to do on their truck.
- Az-Kickin **Out of business**