Brake issue

[cj847]

I have a problem I am having trouble diagnosing. I'll try to keep this short enough so that folks will read it.

1967 C10 with CPP front disk kit (installed by previous owner), new cpp booster and master cyl but the old combination valve. Meanwhile my rear drum started dripping (wheel cyl).

Long story short, I installed ALL new rear drum brakes (shoes, hardware, wheel cyl, adjusters, adjuster hdwe, drums). I also installed a new Summit PV2 disc/drum combination valve.

Everything went together and bled well (yes, I used the combination valve bleeder tool).

Went for a test drive and brakes are great. Went across town and brakes got better and better. Then I started noticing I wasn't coasting as well as normal, then the transmission started to kick down early.

THE PROBLEM
As I drive the truck, the rear drums are tightening up. It is like the self-adjusters are adjusting every time I use the brakes. (I am pretty sure they are only supposed to adjust when applied in reverse).
I took the drums back off and double checked that the adjuster and adjuster hardware are on the correct sides, and they are.
It is interesting that the drums were easy to pull off (ie as if the adjuster is NOT the problem).

Any ideas?

[Sheepdip]

Check the brake flex hose that drops to your rear end, sounds like it may be old and acting like a check valve

[pjmoreland]

Is the brake pedal rod that pushes on the back of the master cylinder retracting all the way with a little bit of clearance when not pressing on the pedal? If the master cylinder isn't being allowed to retract all the way, then the brake lines will stay pressurized when not pressing the pedal.

[cj847]

Originally Posted by Sheepdip

Check the brake flex hose that drops to your rear end, sounds like it may be old and acting like a check valve

The rear hose is about a year old. I replaced it when I first got the truck.

But with the quality of parts these days....??

[cj847]

Originally Posted by pjmoreland

Is the brake pedal rod that pushes on the back of the master cylinder retracting all the way with a little bit of clearance when not pressing on the pedal? If the master cylinder isn't being allowed to retract all the way, then the brake lines will stay pressurized when not pressing the pedal.

I am thinking it is something like that, but there is clearance. I am going to investigate more today.

BRAKE ENGINEERING QUESTIONS
The booster - it doesn't matter how much vacuum it gets as long as it's enough, right? (ie 15" mercury vs 17"). It shouldn't affect pedal throw anyway?

If the combination valve was mislabeled and is actually disc/disc wouldn't the brakes drag when cold as well as hot?

[body bolt]

Disconnect the vacuum source from the booster.

[cj847]

I shortened the rod (a little too much) and it solved the issue (at least for the 15 min I drove the truck. Thanks for the input.


NOW the reason I started working on the brakes in the first place.
I feel like my front brakes are not working properly. That is why I installed the correct combination valve.

In an emergency stop situation, the rear brakes lock up immediately and I cannot get the fronts to lock no matter how hard I press. Note there is a new front disk brake kit (installed by previous owner) with new lines, hoses, calipers etc. They work good enough that they get hot and they bled fine when I installed the new combination valve. I am looking for suggestions before I start throwing parts at it.

[dmjlambert]

I think you would have to put 1000 lbs (1/2 ton) of something in the bed of your truck and then see what locks up on an emergency stop. The brakes were designed to stop the truck most efficiently when operating loaded. If it is light weight in the back, those back brakes are going to lock up first rather than force the back tires down into the street as it would if you had a load.

Post a picture of your original combination valve. If it is original to your truck it is not a combination valve but rather a plain distribution block and brake warning switch made for 4 drum brake setup. If it is original to the era but not your specific truck and came from a 71 or 72 C10 then it would be a combination valve, which includes a proportioning valve for the rear brakes.

[Jason Banks]

I put one of these on my 1968 that is converted to front disc:

https://www.summitracing.com/parts/sum-g3905

Installed in the rear brake line. Seems to help. My main problem was gear oil leaking from rear end, into one of the rear drums.

I have the wilwood adjustable proportioning valve on another truck. It's basically the same things but built into the prop valve.

https://www.summitracing.com/parts/wil-260-11179