Rear brake issue

[Chris65GMC]

This is my first old truck and I'm not sure how it should drive but the rear brakes lock up under anything but gentle braking. I have 4 drums a dual master and power brakes from a 70's truck( I think ). I adjusted the front to get 1.5 turns to stop and the problem persists. Think this set up should have a proportioning valve? When the rears lock they unlock when I let off the brake to be clear.

Thanks!

Chris

[62gmc910]

the rear brakes may have the wrong wheel cylinders (too big) or the shoes could have been put in in the wrong order, there is some brake info in the FAC section, it's been a long time since I worked on 4 wheel drums

[Big Fenders]

I had the same problem with my 63 1/2 ton. I had cheap mail order rear shoes that just grabbed with moderate pressure. Same as you with a dual master cylinder[drum/drum]. Switched to Wagner Thermo Quiet shoes with great results. Make sure shoes assembled in correct position[I think small shoe toward the front] Springs and star adjuster also. Shop manual is a big help.
Bob

[Chris65GMC]

Thanks for the help guys. I put a proportioning valve on today, and the symptoms whent away with some adjusting. I'm just a little worried that the propotioning valve may be band-aiding the problem, or worst just limiting the amount of braking power i have. I did take a drum off in the rear and the shoes were on the correct way. Maybe ill change the shoes since it will need to be done adventualy anyway.

Anyone else running a proportioning valve on a Drum/drum set up?

Thanks,

Chris

[Captainfab]

My '66 Suburban still has 4 wheel drums. I'm running a '67-'70 master cylinder with a vacuum booster. I don't know if I would exactly call it a proportioning valve, but there is one of sorts right at the master. If the master you have isn't for a drum/drum system, that may be part of the problem. The proportioning valve you installed, is it for a drum/drum system or a disc/drum system?

[Chris65GMC]

Originally Posted by Captainfab

The proportioning valve you installed, is it for a drum/drum system or a disc/drum system?

I didn't know there was a difference. I think they all do the same job. The valve i put on is adjustable so i should work with both.

-Chris

[62gmc910]

The factory "proportioning" valve is more of a panic stop valve, it is metered to limit rear brakes in an emergency "slam the pedal stop" ,and to activate the brake failure light switch in the event of a line failure. It does very little to help every day proportioning.
Chris65 the dial valve is probably the best solution , But you can help the situation by running the softest ,quality, shoes on the front brakes, soft material provides more friction with less pressure. but if you drive the truck much in heavy stop and go traffic or in steep hills or just want to be able to stop it a disc conversion is the way to go . the FAC section ,I believe has most of the informatio that you would need.

[Chris65GMC]

Originally Posted by Big Fenders

I had the same problem with my 63 1/2 ton. I had cheap mail order rear shoes that just grabbed with moderate pressure. Same as you with a dual master cylinder[drum/drum]. Switched to Wagner Thermo Quiet shoes with great results. Make sure shoes assembled in correct position[I think small shoe toward the front] Springs and star adjuster also. Shop manual is a big help.
Bob

What is considered "cheap" is raybestes a good enough shoe?

[Big Fenders]

Originally Posted by Chris65GMC

What is considered "cheap" is raybestes a good enough shoe?

So cheap,I don't think they had a brand name...Raybestes are good shoes as Wagner.
Good point brought up with the backing plate. Another good reason to have a factory manual is that they show re-assembly. On the backing plate are I think three raised ares where the shoes rub the plate....also where the above mentioned groves would be. Factory manual instructs to put a small dab of grease to prevent shoe hang-up. Both shoes, primary and secondary.
Bob

[aerotruk63]

Could the lines be hooked up backwards on the dual master. The rear brake lines hook up to the front of the master and the front brakes hook up to the rear of the master. So I was told.

[chris91]

On my truck I have the rear brakes on the back(next to firewall, small reservoir), and the front brakes on the larger. I have a dual master cylinder off a '69 chevy C10, no power brakes. I have not had any problems.

[blackedoutharley]

The biggest issue I have seen with drum brakes and people's perception of them is in the maintenance. When replacing the shoes it is important, as stated here already, that the shoes be installed in the correct position. Additionally, I have found that it is very important to be aware that on older vehicles (your truck is at least 40+ years old btw) that any grooves in the backing plates, where the shoes ride, are ground smooth and if need be welded up and recontoured. This keeps the shoes from binding up which may be happening in your case.

When I do the brakes on my '65 C-10 I always replace the hardware kits and turn the drums.

I am fortunate that I have an arcing machine to match the shoes to the drums.

If you haven't taken care of the backing plates though it can cause issues in how well the brakes function.

And I use Raybestos shoes; they are fine for what we need.

YMMV

[Chris65GMC]

Originally Posted by blackedoutharley

any grooves in the backing plates, where the shoes ride, are ground smooth and if need be welded up and recontoured. This keeps the shoes from binding up which may be happening in your case.


YMMV

Thanks, this is a good idea, I know my one backing plate looked banged up fromthe back, so ill have to better inspect the front.

Thanks!

Chris