What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

4wheel discs, FOUND PERFECT MASTER CYLINDER!!

ntsqd said:
GM does an odd thing with brake plumbing that I've not yet figured out or found the reason for. Typically the rear brakes are plumbed with 1/4" tube instead of 3/16" tube. That may not seem like much, but in one foot there is over 2 square inches additional surface area in the 1/4" tube that the 3/16" tube doesn't have. So in a 125" WB vehicle there's roughly 2 square FEET more tube interior surface area for the operating pressure to work against.
I suspect this is done to cause a delay in rear brake operation. For the volumes involved, even with the 14bff's large wheel cyls., it can't be justified by flow requirement.
Click to expand...
Right, and its not a flow optimized system but a static stystem. The size could be driven by economics while staying within performance and liability limits.
 
Been doing some digging and searching here. First I got excited about this new MC that I could swap into the stepside. Now I see that it may not fit into my '78 so well. Am I right to assume that it's not compatible with my stock booster? Does anyone know when they changed the boosters? Would one of the newer boosters swap right into my 78 K10?
 
I just put one of these master cylinders in my suburban today. Mine is a 1989 model and it bolted right up with no problem. The lines hooked up to the proper places too. The one I picked up also had a more normal sized reservoir, not totally huge like in the picture of the GM part. I bought mine at O'Reillys if that makes a difference. I also dissected the factory combination valve and disabled the metering and proportioning functions. I still have to get all of the air out of the system to know how this is gonna work.

*Update* - The one I installed before went bad and I went to O'Reilly to exchange it. Come to find out the one they gave me to start with was for a 2 wheel disc truck. I exchanged it for the 4 wheel disc model, which did in fact have the huge reservoir. After installing this MC I had better brakes, although I do have to push the pedal kinda hard. I believe this could be due to the vacuum booster not working as well against the larger MC. The correct model number was 10-2754.
 
Last edited:
Oxyacetylene said:
I just put one of these master cylinders in my suburban today. Mine is a 1989 model and it bolted right up with no problem. The lines hooked up to the proper places too. The one I picked up also had a more normal sized reservoir, not totally huge like in the picture of the GM part. I bought mine at O'Reillys if that makes a difference. I also dissected the factory combination valve and disabled the metering and proportioning functions. I still have to get all of the air out of the system to know how this is gonna work.
Click to expand...

If you disabled the metering and proporting functions of the combo valve, couldn't you have just used the stock m/c?
I figure the reason everybody is having so many problems with rear wheel lockup is because the combo valve thinks there are still drums in the rear and sends the fluid there first. If you disable that function, then I would think it would act like a normal system again since the m/c pistons are the same size and travel the same distance.

Yes? No? :thinking:
 
GW2k, Are you saying that the stock combo valve has a delay valve built into it? First I've heard of this, but with that and $6 you can get a Starbucks fu-fu coffee.

I've not had one that I could dissect, so I've just assumed they were a prop valve and the shuttle valve/warning light switch.
 
i never had any issues with it at all. i ran it on my 750 miles trip to the beach and back and never had a bit of problems.
 
So its working and you didnt have to mess with the stock proportioning valve. Sounds like a winner!
 
ntsqd said:
GW2k, Are you saying that the stock combo valve has a delay valve built into it? First I've heard of this, but with that and $6 you can get a Starbucks fu-fu coffee.

I've not had one that I could dissect, so I've just assumed they were a prop valve and the shuttle valve/warning light switch.
Click to expand...

Exactly right. The stock valve isn't just a proportioning valve, it's a combination valve. Drum brakes require less pressure but more volume and have more distance to take up before engagement, so the valve not only regulates the pressure but also delays engagement on the disc system.

Here's a full explanation:
http://auto.howstuffworks.com/master-brake2.htm

What I'm going to try is running no combo valve and using 3/4-ton calipers on the front and 1/2-ton calipers on the rear. If that doesn't work, I'll reverse the calipers and see if it helps.
 
I knew that the combo valves had both a prop valve & the shuttle pin switch, didn't know that they also have a delay valve in them. I was aware of GM using a delay valve back in the early days of front discs. MISF's '69 Gran Prix had one. It's removal didn't appear to have any effect. I didn't know that they'd migrated them into the combo valve. The vehicles that I've converted to front discs obviously never had them.

The downside to having worked in the racing brakes biz is that you don't get any exposure to the peculuarities of drum brakes or disc/drum combinations.

hum, more pondering required........

If the 3/4t and the 1/2t calipers use the same pad then they are, according to every brake parts book I own (Autospecialty & Raybestos) or have looked thru, the same size piston: 2-15/16" I have heard of a slightly larger than 3" piston version, but I have never (& I have looked) found a listing.
I think there is an aluminum aftermarket replacement caliper with a bigger piston & I know there is a two piston varient made by Howe (I think it's Howe). I would not use an aluminum caliper of this type on a truck. They are made first to be light and then to be brakes.
 
Last edited:
454k30 said:
So its working and you didnt have to mess with the stock proportioning valve. Sounds like a winner!
Click to expand...

i didnt fiddle with anything else besides getting stainless steel brakelines from shane on here. never had any issues with it at all.
 
ntsqd said:
If the 3/4t and the 1/2t calipers use the same pad then they are, according to every brake parts book I own (Autospecialty & Raybestos) or have looked thru, the same size piston: 2-15/16" I have heard of a slightly larger than 3" piston version, but I have never (& I have looked) found a listing.
Click to expand...

JB6 and JB7 brake systems don't use the same pads but a 3/4-ton could have either system in them.
JB6 uses a standard D52 pad but JB7 uses the larger D153 pad and has a bigger piston.
Both have the same mounting bolts/pins and either will fit on the 3/4-ton backing plate.
 
The stock combination valve has a metering function, because from the factory the vehicle has drum brakes. This sounded confusing but made more sense when I took mine apart. The metering valve only affects the front brakes, not the back. The valve holds off pressure on the front calipers until the front line builds pressure (around 100psi?) then it opens and lets the fluid flow to the front calipers. This is to give the factory drum rears time to overcome the springs holding the pads back so that the backs and fronts engage at roughly the same time. The back brakes part of the combination valve only does proportioning, to limit the overall pressure to the back brakes. Two benefits of taking mine apart and disabling these features are: 1. No metering on the front, they begin to engage immediately 2. Now I can add an adjustable proportioning valve to the rear and it won't be affected by what the factory valve was doing. The stock booster is working fine. That's all it does is help you push the pedal and the bigger MC feels fine with the stock booster. The stock metering function probably doesn't make much difference since it only takes a little pressure to activate anyway so it may not be worth messing with. I took mine apart when I was troubleshooting the brake system, but it wasn't the real culprit in the end.
 
haha sweet, yesterday i was looking for this thread for hours....thank god...i think i might go get one for my discs....o ya, did it help a squooshy pedal if you have one?...i cant seem to get rid of mine...
 
goldwing2000 said:
JB6 and JB7 brake systems don't use the same pads but a 3/4-ton could have either system in them.
JB6 uses a standard D52 pad but JB7 uses the larger D153 pad and has a bigger piston.
Both have the same mounting bolts/pins and either will fit on the 3/4-ton backing plate.
Click to expand...
Interesting, with the different pad numbers I had discarded the JB7 option as possibly uncompatible. I need to go to my friendly brake parts store & have them pull one of each. Wouldn't mind going up when I switch to HB.
 
shaun89 said:
haha sweet, yesterday i was looking for this thread for hours....thank god...i think i might go get one for my discs....o ya, did it help a squooshy pedal if you have one?...i cant seem to get rid of mine...
Click to expand...
Honestly I doubt the MC will really help with a squishy pedal. I thought it would help mine, but the real problem was air in the rear calipers. Try pumping the pedal with the engine off. If the pedal gets more firm then you might have air in the system still. If you pump it and get a firm pedal but it leaks down if you hold it then your stock MC could be bad.

My D60 front has the JB7 brakes and the pads are noticeably larger than the JB5.
 
You must log in or register to reply here.

Latest Posts

Top Bottom