Power brakes problem?

[kynimrod]

I have been struggling with a power brake problem now for a long while, so I thought I might put it out here and see if anyone can help solve it.
1968 C10 50th anniversary edition with power drum brakes. When I first start and drive the truck, the first couple of times I stop I can really feel the power brakes. As I continue to drive it, the brakes get really hard(no more power assist). I replaced the booster and check valve last year and it made no difference. I checked the vacuum and it is 18 and idle and goes up some when rpm increases. I have adjusted, cleaned and checked brakes on all 4 wheels.

What is my problem? The truck came with power brakes!
Anyone shed some light on this?

[kynimrod]

any thoughts?
ttt

[Eddie H.]

It sounds like the shoes need to be replaced and the drums machined. Most likely they are glazed and fading out once they heat up a little.

[kynimrod]

Good thought...........but the drums were in great shape and the shoes were too! Any other ideas?

[wilbilt]

If the pedal gets hard, it sounds like you are losing vacuum to the booster somehow. If the shoes were getting hot and fading, the pedal would still feel the same and you would likely smell the hot brakes.

Is it possible the vacuum hose to the booster is collapsing when it gets warm? Is it the correct type of hose (i.e., vacuum hose and not a piece of fuel hose)?

[68gmsee]

Barring any mechanical parts being defective and if the brakes work good and then fail after they're used for a couple of times, then as Eddie mentioned fading is the likely culprit.

There are some good articles on the internet on the definition but basically it's brake shoe heating to the point of failure. The rubber hoses (three of them) may not be allowing the shoes to retract all the way and causing overheating. A good way to check is to run the truck until it fails and feel the drums. If they're extremely hot to the touch, you've got heat caused fading.

If the above checks out and they're not overheating, then the only thing left is the master cylinder.

[Crabass]

Not all brake pads/shoes are created equal. The material that the linings are made of varies with manufacturer and product line, and that seems to translate into significant differences in performance. My experience has been that better brakes are more expensive. I use Wagner Thermo-Quiet and have been very pleased-----not that much more expensive and have always been an improvement not only in stopping power but in smooth, predictable modulation as well (disc/drum systems). Have switched six of my seven registered vehicles. Cured the fading problem in my '65 Chevelle non-power drum brake car, although the pedal effort seems to be about the same. Just sharing my personal experience.
That your brakes work until they heat up suggests to me a fade condition. How quickly they fade is largely a function of the lining material composition. Unless you know for sure that you have high quality linings, I would try a set of the better ones (a relatively inexpensive and simple fix if that turned out to be the problem). Before that, though, make sure that your adjustment is good so that you don't have excessive heat from dragging brakes.
My troubleshooting method is to start with the easiest and work toward the more complex, if I stray from that approach I usually regret it.