What is the best way to diagnose a failing Master cylinder

64BROWNIE

MEMBER
1992 OLDSMOBILE 88
3.8L
6 CYL
FWD
AUTOMATIC
160,000 MILES

I recently replaced brake line that rusted in two going from a coupling under the driver seat all the way back to the right rear wheel cylinder. The bleeder screw broke off so I replaced the wheel cylinder at that time also. I bled the brakes using the two-person pumping method. Braking was greatly improved but with greater pedal travel than before the rust-through. After a few weeks I noticed the pedal travel increasing, so I crawled underneath and found a slow leak at a new coupling which I loosened and re-tightened and it seemed to stop the leak. The fluid in the master cylinder had gotten very low and possibly introduced air into the lines so I again bled the brakes using the two-person method. This did not improve the extensive pedal travel very much, but I had a little improvement. I then attempted gravity bleeding from the farthest wheel which happens to be the one where I replaced the wheel cylinder. The clear hose I used would show clean fluid go through a few inches, then a bunch of air bubbles, then clear fluid, then air. This continued for at least thirty to forty five minutes and the air bubbles kept coming. I went through at least a pint of brake fluid and the pedal travel is the same. It is not all the way to the floor, but is excessive. I have examined the calipers, hoses, lines, connections, wheel cylinders and do not see a leak elsewhere. Is there a surefire way to judge whether or not the master cylinder is causing this before I throw money at a part that may or may not solve the problem?

3 Replies

CJ MEDEVAC

MECHANIC
11,005 POSTS

As a review,

Is this the exact way you did the two man bleeding? (look at each of my steps, did yours vary?)

See my answers in this link

https://www.2carpros.com/questions/1980-jeep-cj7-cj-brake-problem

Any good news?

The Medic

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Wednesday, October 19th, 2016 AT 7:43 PM

64BROWNIE

MEMBER
2 POSTS

This is exactly how I did the bleeding with two people. As this did give me some slight improvement, there was still way too much pedal travel so I gravity bled them and saw no change in the pedal travel. I also could never get the air bubbles to stop coming out of the hose. Still too much travel

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Thursday, October 20th, 2016 AT 6:38 PM

CJ MEDEVAC

MECHANIC
11,005 POSTS

It is possible the master cylinder is shot.

If it were good before, with a full pedal, Then suddenly it was going to the floor, the piston traveled into the "not used" portion of the cylinder (maybe it's not smooth inside anymore/ buildup/ basically 'barnacles' eating on the rubber piston) (OR NOT!)

If you let it run out of juice in the master cylinder, it still may have air trapped in it. As I showed in the previous answer, 2 people can "Bench Bleed" on the vehicle. I really like this method! The tubes and adapters needed are cheap at an auto parts store. No need in pumping fast. I sorta pinch the tubes with my fingers sorta like a 'check valve' (fluid/ air out into the reservoir-won't back up). This makes it go faster! (the air can only come out, not move back and forth in the tube) This back pressure usually resulted in blowing the tubes off of the adapters. EZ fix! Wrap 2-3 wraps of string tight over the tubing, over the adapter.

There may be better videos on you tube, I liked this one (in previous answer)

https://www.youtube.com/watch?v=i3Jhbz4mVKw&feature=related

Another possibility for a low pedal/ a pedal needing 1/ 1-1/2/ 2 pumps to get it high and tight each time it's used, may be the brake shoes need adjusting closer to the drum. Sometimes the self adjusters get dirty/ rusty and quit adjusting. You may have to replace/ remove and clean/ or use your brake spoon to get 'em closer to the drum surface.

What happens is the 1st pump may take you closer to the floor. This gets the shoes to the drum (pushing a lot of brake fluid to travel that far) The quick pump, which gives you the higher pedal (2nd pump) now only used a little fluid, as you "grabbed it" before the brake shoe springs could have time to shove the brake fluid back into the master cylinder from the 1st pump. Adjusted out properly only uses a little fluid to move the shoes just a little bit, resulting in a higher pedal all of the time. Calipers on the front end do not have this kinda problem.

Did I make this understandable?

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The Medic

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