Not entirely OT...truck brakes

Dave H (MI) · Nov 22, 2013

My 1976 Chebbie C20 (the tractor hauler) pulled a new one on me today. Pulled up to an intersection and the brakes felt weird, pedal went slowly to the floor and she kept a-rollin. Lift up foot again and depress pedal and she stops great. Works for awhile and then does it again. Brakes have about 10K miles on them and the master cylinder is full. No drips (thus the full MC). Scratchin my head on this one. Any ideas?

bo · Nov 22, 2013

Master cylinder is failing.

gmccool · Nov 22, 2013

Master cylinder is going bad.

Iowa Barber · Nov 22, 2013

Master cylinder by-passing internaly/replace

guido · Nov 22, 2013

Hello Dave H,

If no work has been done and it just happened, then the master cylinder is your problem.
You can do a quick check, and see if you have play at your pedal for the master cylinder rod. You should have about 1/8 in play before the rod hit the cylinder, or it will act just like you described...............

Guido.

ShadetreeRet · Nov 22, 2013

There are not too many "ifs" in a situation like this. Guido has a good point, if the M/C has been replaced there is a possibility that the rod does not have enough slack inside the piston. Other than that, if the pedal goes to the floor, or leaks down with your foot on it, the seals on the pistons are bad and the M/C will have to be rebuilt or replaced. The cheaper, "reconditioned" units have a higher failure rate, or at least that's my experience.

Farmboy856 · Nov 22, 2013

Yes, replace the master cylinder immediately. I had a 1975 Olds 98 which I bought from a neighbor lady's estate in 1996. The car had 10k miles on it. Like brand new. Coming out of the parking garage at work one evening, my boss was in front of me waiting to pay to exit. I nearly creamed my boss's new 7 series BMW due to the same problem. I got her stopped about an inch from his rear bumper. He would have killed me.

NCWayne · Nov 22, 2013

Gonna chime in late, and go with the rest, in saying you got a master cylinder going bad, so it needs to be replaced.

Owen Aaland · Nov 22, 2013

If the fluid level does not drop in the system the only thing that can be the cause is the master cylinder.

Many times when they start to fail like that if you really hold a lot of pressure on the pedal it will not drop to the floor. The piston cups get worn and fluid will leak around them under light load. Heavy pressure forces the cup seals into the cylinder bore and slows or stops the leakage.

John B. · Nov 22, 2013

What you've described I would say it's your master cylinder that is bad.

Determined · Nov 22, 2013

A bad front wheel bearing will do what you have described.
You turn a corner and the rotor woll flop around and push on the brake pads forcing the caliper piston back into its bore.
It will take several applications of the pedal to move enough fluid to position the piston back to where it should be.
Dave

old · Nov 22, 2013

Master cylinder is going bad. BTDT on my 1980 Chev heavy half and got lucky when it did that I was just around the corner from O'Reilly's. I was on my way to an auction where I knew I was going to haul some heavy stuff home so swung around and got a new master cycle installed it and been fie ever since

wd45man · Nov 22, 2013

had the same problem with my impala, found that the front brake pads on both wheels had seperated from the backing plate, replaced with new pads, problem solved.

Not entirely OT...truck brakes

Dave H (MI)

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bo

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gmccool

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Iowa Barber

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guido

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ShadetreeRet

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Farmboy856

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NCWayne

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Owen Aaland

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John B.

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Determined

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old

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wd45man

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