Bad master cylinder, or re-bleed needed? (new booster fixed)

[LeSabre in Buffalo]

I had some time in between jobs today, so I decided to bed in my new rotors and pads by doing some 60-20 mph stops. The ABS kicked in on the first stop. On the second stop, the pedal gets firm for a second, then continues down near the floor. Stops after that resulted in a very soft brake pedal, and not much stopping power. The pedal comes back up, only to get near the floor when stopping. The car will stop, as long as there's enough room and I'm standing on the brake pedal.

Parts replaced in the past year:
Wheel cylinders
Rear brake shoes/hardware
Front rotors/pads
All 4 rubber brake hoses

Brake lines are rust-free, as they were replaced a little over a year ago.

What do you folks have to say? Re-bleed, or master cylinder time?

[00Beast]

I would re-bleed first, then see what happens, but there's a high likelyhood of it being the MC.

[LeSabre in Buffalo]

Should mention, at the time of the hoses I kicked in the ABS repeatedly on a gravel road. The pedal felt a bit soft right after that, but got better. I haven't re-bled the system since then.

Fluid level looks fine, and there's no fluid in the vacuum hose from the booster.

[LeSabre in Buffalo]

Spent the afternoon bleeding the brakes. Bled them twice, once by myself, and once with a neighbor. After a liter of brake fluid, the pedal still sinks to the floor. No air or leaks we can see. Time to bench-bleed the remanufactured master cylinder that was all that was available locally.

[00Beast]

Re-mans are better than new, Dave. Re-mans are OE parts that have new pistons, seals, etc. New parts are of cheap Chinese quality. There's a reason re-mans cost more.

GL w/ the installation!!

[LeSabre in Buffalo]

Thanks, Ed! I'm bench-bleeding it in a little bit, then onto the car it goes. I'll bleed it tomorrow when my neighbor and I won't be freezing as quickly. I owe that guy a beer.

[Jrs3800]

Master Cylinder failures are common place with our 92-99 H Bodies... Good luck with it Dave...

[LeSabre in Buffalo]

Thanks, Don!

My neighbor and I replaced it and ran about a liter of brand-new fluid through it the old-fashioned way, going longest to shortest brake line. The car stops a lot better, but the pedal is still near the floor when the car stops. It's also quite soft when the car's on. The pedal will sit up high and be stiff as heck when the car is off and the vacuum booster is depleted. I'm letting it sit with the nose down for a little bit, then will drive it around to see if it will get better.

Besides re-bleeding, any other suggestions for getting the brake pedal back up where it should be?

[LeSabre in Buffalo]

I capped off the vacuum line to the brake booster, and drove around the block very slowly. The brakes stop the car fine with lots of effort. No loss of brake fluid, either. As soon as the booster is hooked up, the pedal is back onto the floor when trying to stop.

I'm leaning toward the brake booster at this point.

[00Beast]

Kinda sounds like you got a bad booster Dave.

[spoiledred94]

I'd like to know if that fixes your problem Dave. I have a similar problem Pedal fades almost to the floor when pushed very hard. Re-bled several times and replaced the MC as well as flushed the lines and replaced rear hardware and rebuilt front calipers. They are adequate now but I still they think it could be better.

[Jrs3800]

Its possible for it to be a bad booster... Are the rear brakes adjusted up so that there is a small amount of drag on the rear drums? Also rebleed the ABS pump if at all possible...

Another question.... When you have the car running and the pedal goes to the floor, does the cars idle change at all, and if it does is it a little or to the point where the car feels like it may want to stall out... Do you hear any hissing from the booster?

[LeSabre in Buffalo]

Rear brakes are adjusted properly. I looked in the FSM and saw nothing on how to bleed the ABS pump except by the scan tool.

The car doesn't want to stall out, and there's no change in the idle that I've noted. There might be a slight hissing, I'll have to listen closer the next time I'm in the car. I'm remembering a little hissing, but that could be my memory playing a trick on me.

[Jrs3800]

I was thinking more along the lines of under the hood... Only thing I can think of off the top of my head is that you may still have air in the system..

[LeSabre in Buffalo]

The booster is hissing slightly at the brake pedal when the pedal is depressed and held. In searching around the internets, one guy had a similar problem with the booster on his Ford F250.

Ordered a booster, should be here in a few days. Of course nobody stocks one around here. Until then, I'll have to be extra careful braking.

[Mechanical Mike]

I've only had one vehicle that needed to have the brake booster replaced. It was a 99 Tahoe & strangely enough it went through 2 boosters from 99 to 2005. Both times when the booster would start leaking vacuum it would feel like the brake pedal was pushing back against your foot.

[renchjeep]

Re-mans are better than new, Dave. Re-mans are OE parts that have new pistons, seals, etc. New parts are of cheap Chinese quality. There's a reason re-mans cost more.

GL w/ the installation!!

Funny, I have had the exact opposite experience. I have (in the past) installed several "rebuilt" masters and wheel cylinders on MANY cars/trucks, only to find them leaking very soon. Now, I swear by only NEW units.

Just recently, my brake mc went out at 201k, and I replaced it with a new O'Reilly/Autozone (IIRC) unit for $39.95. Filled it with Prestone DOT 3 Synthetic fluid, bench bled, then bled the entire system (starting farthest away, going to nearest), bleeding until I got nice new fluid out, then some more. I did all this with the key in the "off" postion, in order to not activate the ABS pump.

End result, the car stops better than ever! After 4,000 miles, however, the pedal was sinking a bit more, and I needed to re-adjust my rear brakes. Pedal came right back! Guess it's time for some new self-adjusters. Next project on the board.

Like I needed another one!

[LeSabre in Buffalo]

But why would the pedal be high and hard to push when the brake booster vacuum assist is depleted, and sink right to the floor as soon as vacuum is applied to the booster? The brakes will stop the car with the brake booster disconnected. The pedal is still far off the floor in that case.

After bleeding the system and installing the properly bench bled remanned MC, the same symptoms persist. The drums hang a little bit when rotating the tire. I still can adjust them and see where that gets me.

[LeSabre in Buffalo]

Noticed this today: All the brakes are very hot after a short drive at low speeds with minimal braking, and my recently installed front pads are quite chewed up. Some heat is normal, but not all the brakes being warm to the touch after a short drive. Especially the rear drums.

Brake booster ordered, and is on the way.

[00Beast]

Hmmm. Are your calipers sticking maybe? Look closely at the pads, is the inside one maybe a little more worn than the outers?