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Discussion Starter #1
I've had this happen the last two time I drove car over 10 minutes or so. When I start off brakes are fine but then the pedal gets real firm and the brakes are dragging. If I let off gas the car stops quickly. I can let it sit for just a few minutes, then restart and pedal is fine.
I've checked the vacuum line from manifold to brake booster and it's connected and in good condition.
Thanks
Bob

1976 el caminio w/power brakes
 

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Hello Bob:

It sounds like you have already done some homework and it's not always an easy issue to solve. However, I researched online and found a list of things to check in this article that perhaps might help. It's interesting reading and I learned a few things I didn't know or understand about brake systems and how brake boosters work.

https://techtalk.mpbrakes.com/how-to-series/how-to-diagnosing-a-hard-brake-pedal

Good luck.

Bruce
 

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If your front brakes are dragging, I would suspect the front brake lines collapsing inside and not allowing fluid to return.
 

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It sounds to me like something is pre-loading the master cylinder. If something is pushing in on the MC piston, it will do just that. check that you have full return on the brake pedal. Take a wrench with you on a drive- when it hardens up, loosen the nuts a little bit that hold the MC to the booster.(unload) if your pedal returns, that's the deal. Good luck!
 

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X3 on the front flexible rubber brake line(s) collapsing. They fail inside and fluid is pushed between the inner and outer part of the rubber line when brakes are applied. Pressure effectively traps brake fluid from returning to the master cylinder after brakes are applied. When it sits awhile the fluid eventually bleeds back and relieves pressure. Very common problem and very easy fix.
 

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X4 Replace the rear hose as well
 

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Discussion Starter #8
It sounds to me like something is pre-loading the master cylinder. If something is pushing in on the MC piston, it will do just that. check that you have full return on the brake pedal. Take a wrench with you on a drive- when it hardens up, loosen the nuts a little bit that hold the MC to the booster.(unload) if your pedal returns, that's the deal. Good luck!
I believe it's all 4 wheels that are braking so I'm going to try backing off the 2 nuts the next time it happens.
thanks to everyone for all the suggestions.
Bob
 

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Discussion Starter #9
It sounds to me like something is pre-loading the master cylinder. If something is pushing in on the MC piston, it will do just that. check that you have full return on the brake pedal. Take a wrench with you on a drive- when it hardens up, loosen the nuts a little bit that hold the MC to the booster.(unload) if your pedal returns, that's the deal. Good luck!
Today the pedal got firm again and I was just pulling in the garage, I got out and all 4 wheels/brakes smelled hot, I then loosened the 2 bolts holding the master cylinder to booster and the pedal loosened up (went down) and brakes freed up. Is there an adjustment on the pedal to give more free travel?
thanks
Bob

I had also tested the hose and check valve. Hose is correct size and in good shape. Check valve tested ok.
 

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Discussion Starter #11
I adjusted (shortened) the rod on master cylinder about 1/16" hopping to relieve load. I drove for about 15 minutes a couple of days ago and it seemed fine. Today after about 15 minutes I could smell brakes but they didn't seem to be dragging as much as before. I pulled in garage and could see smoke coming from both rear wheels. Attached are a few pics of disc set-up on rear wheels (some kind of aftermarket set-up).
Could I need to adjust master cylinder rod more or is this a rear wheel only problem?
thanks
Bob
 

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Director Region 19 Owner of the KLRMino
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I adjusted (shortened) the rod on master cylinder about 1/16" hopping to relieve load. I drove for about 15 minutes a couple of days ago and it seemed fine. Today after about 15 minutes I could smell brakes but they didn't seem to be dragging as much as before. I pulled in garage and could see smoke coming from both rear wheels. Attached are a few pics of disc set-up on rear wheels (some kind of aftermarket set-up).
Could I need to adjust master cylinder rod more or is this a rear wheel only problem?
thanks
Bob
Since you have a rear disc conversion. I wonder if PO installed the correct proportional valve for disc to disc and not the stock disc to drum
 
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