14 Bolt Disc Brake Issue

[Mark Leisenfelder]

Ive had rear discs on my truck for over a year now, and have about 15k miles on the rear brakes. I have an adjustable proportioning valve. I had a squeaking noise coming from my rear brakes the other day so I took the wheels off to investigate, and they pads and rotor look like they have 60k miles on them, I'm assuming they have been dragging since the install. What would make the pads drag on the rotors and cause this?

 

[Fastereddie]

Did you remove the residual pressure valve?

 

[Mark Leisenfelder]

apparently not. where would I find this residual pressure valve and what does it do.

 

[Fastereddie]

It is down on the stock proportioning valve behind the crossmember where the rear line comes out of it.
It maintains 3-5 psi of pressure to the rear drum wheel cylinders.

 

[Mark Leisenfelder]

hmmm how do I remove it

 

[sweetk30]

remove the whole thing and replace with a tee for the front . then straight line from master threw your adj valve and to rear .

or get fancy and do this next time . https://www.summitracing.com/parts/wil-260-11179/overview/

 

[Mark Leisenfelder]

I would rather just remove the little valve then the entire proportioning valve

 

[AV8TER]

You would be best to remove the entire combination valve than to work on what you have now. There is several things in that thing that you don't want anymore with 4 wheel disc. It has a hold off valve that doesn't let any pressure go to the fronts until they reach somewhere in the area of 100 psi. That is due to the huge springs the stock rear drums use. They (the engineers) let pressure to the rears first to overcome that spring pressure before letting pressure go to the fronts. So, under light breaking, all you use is the rear breaks if you convert to 4 wheel disc. It also has a built in proportioning valve. Not really tuned for 4 wheel disc. And it has the residual valve. Here is a diagram of the combination valve so you can see how complex it is. Just pull it out. The only piece that would be nice to keep is the warning system but anybody with any brains would know if you lost half your breaking system anyways so.....

 

[diesel4me]

I "think" most master cylinders used on disc/drum vehicles had a similar valve installed in the rear brake port on the master cylinder too,if the vehicle had rear drum brakes...maybe GM combined it with the proportioning valve ?...

 

[AV8TER]

No residual valve in master cylinder by this time. They moved it to the 'combination valve'. One more reason to just ditch it.

 

[nvrenuf]

No valves of any kind in my Blazer, drives great.

 

[JoshHefnerX]

Gut that combo valve and you'll be better.

 

[chulisohombre]

I gutted my prop valve and have had zero
Problems 10 years. I left the safety switch in though so I know if I've lost pressure on one side.

 

[Mark Leisenfelder]

I gutted my prop valve and have had zero
Problems 10 years. I left the safety switch in though so I know if I've lost pressure on one side.

so you just undid the inlet front line and removed everything inside, and did the same for the back?

 

[JoshHefnerX]

Remove everything from the combo valve, front and rear. There used to be a writeup on here from 'atron' can't find it anymore. May have been lost..

 

[chulisohombre]

so you just undid the inlet front line and removed everything inside, and did the same for the back?

Yep. Left the floating valve in the center so
It still valves the front from the rear and if there's a leak It will still Let one side still
Have some breaking ability.

 

[datmony]

Like chulishombre notes above, this is super important. Here is another cheap alternative to set it up right. This is a sweet setup from Wilwood that is easy to add into the system.

http://www.jegs.com/i/Wilwood/950/2...pmt_be_pdv_c&ref=paid&brand=&channel=Shopping

 

[Mark Leisenfelder]

The part I'm confused on is I currently have a rear adjustable Prop. valve on the rear brake line, along with the factory prop valve. I understand that drums require more pressure, but once that pressure goes to my adjustable valve doesn't the pressure change based on the where I have it set?

 

[AV8TER]

The part I'm confused on is I currently have a rear adjustable Prop. valve on the rear brake line, along with the factory prop valve. I understand that drums require more pressure, but once that pressure goes to my adjustable valve doesn't the pressure change based on the where I have it set?

Yes, the added valve will function as intended. The problem with keeping the factory combination valve (its not just a proportioning valve) is the way it controls the rear drum brakes. Disc brakes are linear. In other words, the more pressure you put into the caliper the more clamping force it applies to the rotor in direct relation to input pressure. Drum brakes are exponential. They are (at least the type we have on our square bodies) duo-servo or self energizing brakes. They start off needing a lot of pressure to get them going. They have big springs to overcome and take some pressure to start engaging them also. As they start to grab the drum they start to 'wrap up' all by them selves. That's why the rear shoe is longer than the front. The front shoe shoves the big rear shoe into the drum as the drum turns. So, drums require progressively less input pressure to get more stopping power out of them. The stock combo valve is designed to counter for that on the rears. The harder you push on the pedal the more unequal your braking pressure between front and rear gets. The rear starts to drop off a lot compared to front pressure. That's one of the reasons to get that stock combo valve off of there. It really isn't very good for rear disc brakes. It has a residual valve that keeps too much pressure in the rears, a hold off valve that will make the rears apply quite a bit before the fronts and a proportioning valve that has a really screwy profile for disc brakes.

 

[datmony]

av8ter is spot on.

And it is the residual valve aspect of the stock combo valve that is the culprit in your premature wear. What it is doing is keeping the brakes engaged ever so slightly even with no pressure from the pedal and they just wear and wear and wear because they are never disengaging completely.

Pay far less attention to the pressure than you are. In the example of the adjustable prop valve for the rear all that you are doing is taking full pressure (pressure at the master cylinder) and reducing it to create the correct behavior compared to the front brakes. If you are in a panic stop situation, you don't want the rear to lock before the fronts. That is quite simply all that adjustable valve is going to do for you. Reason to have an adjustable one is simple...... you are creating a Frankenstein system. You are pairing together components that weren't designed together and the prop valve will simply allow you to decrease pressure going to the rear enough to "tune" the brakes in the system.

Now were this the factory doing this and they were going to reproduce a jazillion of these..... they would engineer that exact pressure amount needed for the proper rear engagement right into the combination valve for simple reasons (both pressure during brake application and residual pressure required for drums to behave correctly) ...... it's cheap, it's repeatable.

Do a little reading on that Wilwood valve I posted above. It is a pretty sweet little setup and solves for these issues. I would simply ditch the existing valves you have (proportioning and combo) and swap that in. Bend up a few lines, flare a few tubes and viola. What you will want to do then is head to a remote road and begin testing. Get a little speed, mob on the brakes..... figure out where the rears lock and adjust the proportioning valve accordingly. Once that is tuned you can all but set it and forget it. I can tell you simply though that if you leave the existing combo valve in there unmodified it will never work correctly. You can modify it to work as others have done but in my view more hassle than it is worth when there are simple aftermarket alternatives for sub $100 shipped.