Can a master cylinder just fail during brake bleeding?

Hi all…was replacing the flex brake lines on both front calipers on my 85 Olds Cutlass Supreme (V8, RWD).

Got the lines replaced, bled the calipers, but it still felt like there was air in the lines so we bled the rear drums and the fronts again. Took it for a quick test drive and the pedal kept going right to the floor, brake light on, and there’s very little braking power. So we bled the master cylinder. The test drive after was a BIT better but still not right. The brake light was off, but when approaching a red light, apply the brake at a regular rate and the brake light comes on, pedal goes to the floor and very little braking power again. Pump the brake pedal quickly and it builds up pressure in the pedal, the light goes away, and braking power comes back to a normal amount. At the next light, same thing happens…pressure seems to go away and comes back after pumping the pedal.

No leaks…we re-bled the master cylinder a couple times with no different results. I’m being told the master cylinder needs to be changed, has failed, but I can’t see how it was working fine for the past year and now all of a sudden, when replacing the rubber flex lines, it fails. Is this possible?

One minor note that may or may not mean anything…during the first master cylinder bleed, after we were done, I was still pumping the brakes with the car running as it felt a bit spongey, and well, someone opened the fluid resovoir for the master cylinder and of course…brake fluid everywhere…we rebled right after, but could this have damaged the master cylinder?

Thanks for any suggestions…


My feeling is that there is still air in the lines. Sometimes fluid may pump out and one assumes there is no air but in actuality there are air pockets remaining. Even when fluid squirts out keep bleeding it.

Also, when bleeding always start at the wheel fartherest away from the master cylinder and work your way around; RR, LR, RF, and then LF.

It’s possible to over stroke the master cylinder when bleeding. Short strokes work best.

You could bleed the brake master cylinder, DIRECTLY. That is, disconnect the brake lines at the brake master cylinder. Get metal brake lines from an auto parts store, or from the brake master cylinder replacement package (box). Cut to a length so that the lines attach to the brake master cylinder, and curve back to the brake master cylinder reservoir. Fill the reservoir, and slowly pump the brake pedal until no bubbles come from the ends of the bleeder lines. Remove the bleed lines and connect the regular brake lines. Bleed each brake starting at the wheel farthest from the master cylinder. // To bleed, attach a (fish tank) clear hose over the bleeder nipple. Hold the clear hose in an upward arc and loosen the bleed screw 1/3 turn, and have your helper slowly push and release the brake pedal until there are no more bubbles (repeatedly loosening and tightening the bleeder screw is NOT necessary). Go to next wheel and repeat the procedure. // If the old front brake hoses were swelling, or had interior deterioration (both likely), the rear brake hoses are most likely in the same condition. // The rear brake shoes may not be automatically adjusting, thus, having excessive clearance between the drum(s) and the shoes. This clearance take a lot of brake fluid to close during brake application. Remove the rear brake drums. Adjust the adjuster until the brake drum just slips onto the brake shoes. Bleed and test.

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You can damage the internal seals if you push the pedal further than it is used to going and a crud layer has built up at the normal travel limit.

Thanks for all the suggestions guys.

We did bleed RR, LR, RF, LF before doing the master cylinder, and I wasn’t thrashing the brake pedal when we’d bleed. I have a feeling that TwinTurbo had it right, this looks like the original master cylinder and I don’t know when the last time the pedal was going to the floor as much as it did yesterday, so I’m guessing it was an internal seal that went.

We replaced the master cylinder today after bench bleeding the new one, and also the front calipers as they were really rusty and old…it’s fine now. So, I started with just replacing flex hoses and ended up doing a bit more but I guess that’s how it goes with old cars sometimes.

Thanks again! Until my next “adventure” with the Cutlass…

I’d be surprised if the bleeding sequence isn’t diagonal instead of front to back. LF, RR, RF, LR. Also on masters that are at a slant, you have to either bench bleed level, or jack the back end up to level it. Otherwise air gets trapped in the upper portion of the master.

I had a master cylinder fail when I was doing the final bleed after a brake job. Maybe they just reach a point where they will fail at any time and the stress of the brake job and the bleeding pushes them over the edge.

Thanks keith and bing, I think what happened to you keith, happened to me.

Well, after driving for a bit, I first noticed that the car pulls to the right when braking. It isn’t very pronounced, and after driving a bit today, it seems to be going away. What can cause this? More air? If it’s air, would I be able to just bleed the front right caliper?

Also, the pedal is a lot firmer now, harder to press…I’m kinda used to the old softer feel, is this something that can be adjusted or is it what I have to get used to now with the new MC? It feels a bit too firm…I get the braking power as before but just need to press harder for it.


If the car pulls to the right, when braking, it could be that the LEFT side brake isn’t applying as much braking force as the right side. Make sure that the metal brake line, on the left, isn’t kinked or dented.

One started leaking while bleeding it. Using it is abusing it so I like gravity bleeding. The less wear and tear, the better.

Thanks for the suggestion, I’ll take the wheel off and check the metal lines. What about the firm pedal? Can anything be done about that?

I’m reading on the web about hard brake pedals…thinking of trying to adjust the power booster at the brake pedal linkage for more freeplay to see if that helps. Any comments or warnings before I do that? It sounds like this has to be done sometimes after installing a new MC?

I did test the power booster by pumping the brakes with the car off, then pressing down on the brake pedal and turning the car on…the pedal did sink a few inches when the car turned on, so I’m assuming my booster is working and has vacuum.

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