Rear Brakes Lock Up WAY too soon!

[69GMCLonghorn]

I have a 1-Ton with 4 wheel power drum brakes. In the 6 years I've had it, I haven't been able to solve the problem of why the back end locks up even under light or moderate braking. Here's what's been done so far:

The brake shoes have been changed about 3 times
Drums turned once (still perfect)
Wheel cylinders replaced @ all 4 corners (no leaks)
Master cylinder replaced
Axle seals replaced (I thought maybe a small amt of gear oil might be doing it)

One guy said it felt like the back end was braking like the front end should, as if the lines were swapped. I've looked at it and you couldn't do this without totally rebending the lines. It has a proportioning valve, but there's no adjustment on it.

What I've done that has helped is I installed an inline pressure valve (good old Summit Racing to the rescue!). This has helped, but now the brakes feel funky. If I really hit 'em, there's a point where the pedal just stops and won't go any farther (about 1/2 way to the floor). Could the brake booster be causing any of these problems?

I'm completely out of ideas. Anybody with more brake knowledge than me, please chime in. I'm open for suggestions.

[cdowns]

you could install an adjustable valve in the rear brake line

[WorkinLonghorn]

My Longhorn came (used) with a camper on it and the brakes were perfect.
When I removed the camper I had the same problem as you describe.
Maybe the proportioning valve is calibrated for a heavy load in the back??
When I am driving it empty I lower the tire pressure to 45 psi so that I have a larger contact area. That helps,but then when I load it up I have to add air according to the weight.
I believe the answer is to get an adjustable prop valve, which I've been meaning to do. I hear they have them at Summit and other places.

[69GMCLonghorn]

Actually, I did put a proportioning valve in it and that has improved it (see original text).

Funny you should say that. The old guy that had it before me was the original owner and had a camper on it most of the time so he didn't notice the problem. When he sold the camper, he was like "holy crap! What's wrong with my brakes?!" So I guess the solution is to buy a camper! (not a bad solution, but pricey!)

I feel like if I could get the problem solved, I could stop a FREIGHT TRAIN! There's an incredible amount of braking at the ready. I just need to tap into it sans lock up!

[68gmsee]

Sorta emabarassing when you get to a stop light and everyone freaks out because of the screaming brakes sound. People think you're going to slam into them...

On my 68 GMC I had converted mine to power brakes and after I went back to regular brakes I have no more problems. If I go back to the power brakes, I'll get an adjustable prop valve.

[69GMCLonghorn]

Yeah, well, let me tell you, the "people" ain't the only one who thinks I'm going to slam into them. Many a skidmark has been left due to my brake issues, and I DON'T mean on the pavement! HAHA

[Blazer1970]

You could grab one of those lever operated adjustable proportioning valves that a lot of the newer trucks have on them. The valve is attached to the frame and the lever operates off the differential so that it is load sensitive.

[WorkinLonghorn]

Blazer1970,yeah I heard about those. I wonder what year trucks have them and I would guess that they are calibrated for the spring rate so maybe a 3/4 truck would have different one than "1/2 ton" truck.

[VDOG]

I had the same problem on my 69 1/2 ton with 4 wheel drums, no power.
Everytime I slammed on the brakes I slid forever, changed to disc in the front and have never looked back.

[Blazer1970]

Originally Posted by WorkinLonghorn

Blazer1970,yeah I heard about those. I wonder what year trucks have them and I would guess that they are calibrated for the spring rate so maybe a 3/4 truck would have different one than "1/2 ton" truck.

All you would have to change is the length of the lever arm to calibrate to your application. I know that my brother's '01 Suburban with the big block and heavy duty suspension has one on it. I think a lot of current applications still use this setup.