Brake problem...frustrated

[airbrushguy]

helping my buddy...
It's a 1970 K20, v8, drum brakes, power booster was installed 15 years ago.
Booster replaced by mechanic recently after new shoes, cylinders, MC, and brake lines.
Truck stopped well after brake work a month ago, but now acts like it doesn't even have "power" brakes....must press very hard to stop. Brought back to mechanic but he can't seem to be able to fix. He re-adjusted brakes and was good for a short while, but now is hard to stop again and if you press hard enough, brake light comes on!
Took drums off, linings look good.
Any suggestions?
Thanks

[68gmsee]

Originally Posted by airbrushguy

helping my buddy...
It's a 1970 K20, v8, drum brakes, power booster was installed 15 years ago.
Booster replaced by mechanic recently after new shoes, cylinders, MC, and brake lines.
Truck stopped well after brake work a month ago, but now acts like it doesn't even have "power" brakes....must press very hard to stop. Brought back to mechanic but he can't seem to be able to fix. He re-adjusted brakes and was good for a short while, but now is hard to stop again and if you press hard enough, brake light comes on!
Took drums off, linings look good.
Any suggestions?
Thanks

I would replace the master cylinder. It's the cheapest thing to do.

If the rear brakes are doing most of the stopping, it will be very hard to stop. The MC has two pistons, one for the front brakes and one for the rear brakes. If the piston for the front brakes is bad your braking primarily with the rear brakes.

[ole dollor]

hard peddle sounds like brake booster , or booster not getting proper vacuum...

[airbrushguy]

How do I check if booster is getting proper vacuum? Could it be that the pushrod is just to short?

[DT1]

Originally Posted by airbrushguy

How do I check if booster is getting proper vacuum? Could it be that the pushrod is just to short?

If the brakes worked well before then the booster rod is probably adjusted properly.
I have had poor luck with rebuilt boosters. I installed a rebuilt unit last August and it was bad from the start. I returned it and the replacement worked well until about two months ago, then it also failed. I replaced it under warranty and so far it has been OK.

[airbrushguy]

Originally Posted by DT1

If the brakes worked well before then the booster rod is probably adjusted properly.
I have had poor luck with rebuilt boosters. I installed a rebuilt unit last August and it was bad from the start. I returned it and the replacement worked well until about two months ago, then it also failed. I replaced it under warranty and so far it has been OK.

Good point. He bought the booster from brothers, I think it was new???
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[OJ1988]

Vacuum us the first thing I'd check.
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[slikside]

Kinda curious about that brake light coming on. That should really
never happen. It indicates that one system (front or back) has essentially
failed. Booster problems shouldn't cause that. I'm thinking bad m/c.

One simple test for booster fumctionality: with the motor off, pump up
your pedal til firm and hold. Start the motor. If pedal moves toward
the floor, your booster is working. If not, its dead.
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[speedfreak369]

Possible .....air in the lines
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[airbrushguy]

Originally Posted by OJ1988

Vacuum us the first thing I'd check.
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How do I do that and how much should it be?
I have a vacuum gauge.
Thanks everyone for your help.
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[airbrushguy]

Originally Posted by airbrushguy

how do i do that and how much should it be?
I have a vacuum gauge.
Thanks everyone for your help.
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ttt

[ole dollor]

at idle it should be around 16 inches and hold steady at that...not swing back and forth ....did you try the break peddle test that silkside described ....that will tell you alot.....

[airbrushguy]

Originally Posted by ole dollor

at idle it should be around 16 inches and hold steady at that...not swing back and forth ....did you try the break peddle test that silkside described ....that will tell you alot.....

Pumped on brake pedal while engine was off....pedal didn't come up much but was hard (about 1/2"lower than clutch)..started engine, brake pedal dropped about 3/4-1". Is that enough to prove that booster is good?
Thanks
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[Classic Heartbeat]

The brake light that came on indicates that you have air in the proportioning valve somehow. If the master cylinder had a bubble stuck somewhere it could have come free and worked its way to the propertioning valve. I would rebleed the system and see if that fixes it. WES

[airbrushguy]

Should I rebleed each wheel or just bleed prop block by cracking each fitting while holding pedal down.
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[OJ1988]

Bleed at the wheel starting with the right rear, then left rear, right front, left front.

[ole dollor]

doing the booster test should tell you that the booster is ok,like others said try bleeding all the wheels again and see if that helps....when you drive it do you have to pump the brakes to get a peddle?...or is it hard the first time you hit the brakes....how long does the brake light on the dash stay on when you hit the brakes....does it go off all by itself...

[Murphy IRE]

Good luck, hopefully it's air in the line. but with the pedal being hard, I don't think so. I have very low vacuum on the engine, 11 -13 inches. I have replaced about everything and added a vacuum pump. I got the best brakes I've had , but I still can't do an emergency stop. I am installing a hydraulic power assist sytem. That will put out over 1800 psi to the wheels. I should have it done in the next week or two. I'll let you know how it goes.

[wilbilt]

What quality level are the shoes that were installed? Cheap linings will glaze and overheat, causing poor performance. You might want to check for "hard spots" (blue from heat) in the drums. Hard spots will result in a high pedal, but poor braking.

There is not any remedy other than replacing the drums, as the spots are usually too deep to turn out on a lathe.

[clinebarger]

Originally Posted by wilbilt

What quality level are the shoes that were installed? Cheap linings will glaze and overheat, causing poor performance. You might want to check for "hard spots" (blue from heat) in the drums. Hard spots will result in a high pedal, but poor braking.

There is not any remedy other than replacing the drums, as the spots are usually too deep to turn out on a lathe.

^^I agree. Had that problem with my old '69, Put good Wagner shoe's on it & fixed it.

[airbrushguy]

Drums were good, turned and Wagner shoes used.
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[clinebarger]

Originally Posted by airbrushguy

Drums were good, turned and Wagner shoes used.
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Even a GOOD shoe can overheat & glaze....The "achilles heel" of having drum brakes is they cant dissipate heat very well...Other than that they are actually better than disc brakes as far as friction surface & "cold" stopping power.

[70CHEVYBB]

the red brake light will only come on for two reasons, 1- parking brake pedal has been depressed and 2- loss of hydraulic pressure.

[airbrushguy]

Latest report: bled brakes, rear first, pedal hardly went down when opening rear bleeders, pedal did go down when I bled front.
Driver side cylinder was tough to open, then leaked after bleeding. Going to take drum off tomorrow and probably replace the wheel cylinder to stop leaking.
Still wondering why the pedal didn't hardly go down when I opened the rear bleeders???
Thanks
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[wilbilt]

Originally Posted by airbrushguy

Still wondering why the pedal didn't hardly go down when I opened the rear bleeders???
Thanks
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The piston in the combination valve ("proportioning valve") may be off-center and stuck, blocking fluid flow to the rear brakes. Normally, if the piston is offset, bleeding the opposite end (in this case, the front) will cause it to return to a centered position. There may be a lot of crud in the valve causing it to stick.