Power Drum Brake Problem

[daveo68]

Just got my truck finished after two years. Converted to power brakes right before I parked it. Here is my problem, took it down the road today and when I go to stop ,brakes will not stop gradually. It's like they are stopping at end of pedal travel and front brakes are locking. Pedal is not going all the way to floor. Have replaced M/C, booster, and front brake shoes and hardware, bled front and rear brakes still the same. Also bench bled M/C before install. Have searched all of the posts I could find and still no help. The rubber lines look to be in pretty good shape on the outside. I thought at first it was because I wasn't used to the power brakes but they still should not lock. Any and all advice will be appreciated.
Thanks,
Dave

[krue]

I had the same problem and never did figure it out. I just got tired of fooling with it and went to disc brakes up front.

[sdewolfe]

Dave,

There are several things to check. One, the easiest one, is the adjustment of the pedal rod. The power brake pedal uses less travel than the manual system did. Make sure the pedal travel is somewhere around 3.5" and comes all the way back to the top when released. Related to that is the adjustment of the pushrod between the vacuum can and the master cylinder. The rod length has to be the same as the depth of the seat on the master cylinder. If it is too long it will cause the brakes to drag.

Was the donor a drum/drum setup or a disk/drum setup? I am pretty sure the disk/drum setup has a hold-off valve for the rear brakes that is not needed on the drum/drum setup. Also, a vacuum booster meant for a disk brake application will be too large for a drum brake. System pressure will be too high making the pedal too sensitive. I think that if you go to a larger bore M/C some of that problem can be addressed.

Then there is the vacuum can itself. Was it a junkyard donor? It may be defective. It would take a pressure check of the system to detect that. If it is broken the pressure would go from zero to maximum immediately with no "build up" of system pressure.

If all else fails, you will have to change the pedal ratio. I don't have a manual brake setup to measure so I don't know if the leverage on the pedal for manual is different from power but I suspect that it may be. Maybe someone else knows for sure and can supply that information.

Rubber brake hoses can look pretty good on the outside and be completely or partially blocked from swelling on the inside. I would replace the hoses just as a matter of maintenance. I had a front hose break on my '51 (single circuit system) and it is was quite a wild ride. ;-)

Because the truck sat for two years I would replace the wheel cylinders too.

The power drum brakes on my '70 1/2 ton long bed will sometimes catch me by surprise if I "stab" at the brakes in a semi-panic stop but it is the rear brakes that lockup on my truck. They are just really touchy. New hoses and wheel cylinders helped but I still have to be gentle with the brake application. When the truck is loaded the system is a lot more balanced.

sdewolfe

[airdale94]

My 68 C20 just started doing this. PO converted to power brakes. I'm thinking either rear hose or prop. valve. I'm in the middle of a disc brake swap, so I won't mess with trying to figure it out.

[68gmsee]

sdewolfe gave some good pointers. I converted both of my drum trucks to power brakes and have not had any problems with lock ups unless I step on it hard.

If all the obvious has been checked, then the booster could be the problem.

I know it's simplistic, but this article on how stuff works shows a good detailed description of how the booster works and also shows the parts that are needed to work correctly.

http://auto.howstuffworks.com/power-brake2.htm

Let us know what you find...

Forgot, sdewolfe good first post...

[cdowns]

a combination valve from a disc/drum setup will cause that on drums because of the higher residual pressure to the front brakes

[daveo68]

The truck had a prop valve on it when I bought it 5 years ago. All I did was add booster and changed the push rod on M/C. I think the prop valve is for drum/drum.
Dave

[oldspowered67C10]

Alot of drum brakes have a primary(front) shoe and a secondary(rear) shoe, if you get them mixed up this is often the symptom. Its short lining in the front and long lining in the rear.

[68gmsee]

Forgot to mention. Don't overlook "grabby" brake shoes. Years ago, I bought some that were contaminated and were doing that. I took them back and got a different brand and had no more problems.

The way drum brakes work, the shoes will actually spread when making contact with the drum to increase stopping action. So if they were grabby to begin with, it would accelerate the stopping action.

[daveo68]

I went ahead and ordered disc brake kit from GMCPaul should be good to go now.
Thanks for all the input,
Dave