mtman

abs light came on but brakes still worked fine--jeep is a second vehicle so it was a while before i put some miles on --upon braking now the pedal bottoms out and the engine stalled--searched the internet and found likely cause to be the brake booster--bled the brakes today -but did not help--is there a way to diagnose the booster--i pulled the vacuum line off the manifold and there is a strong vacuum present--i did just change the exhaust manifold and checked for missing or loose vacuum lines--none found--thanks for any ideas on my problem

ol"blue

Welcome to CF. Remove and plug that vacuum line that comes from the manifold and goes to the booster. You want it attached to the manifold with the other end plugged. Start the engine in park, or neutral with wheels blocked, and try the brakes. You should have a hard pedal because of no assist from booster. If the pedal is spongy or goes to the floor, you either have air in the lines, damage/holes to the lines, or a bad master cylinder.

dave1123

I had a weird problem with the brakes on my WJ just recently. The brake pedal would progressivly get lower each time you applied the brakes, but the pedal was firm but low. It also seemed to require more pressure to stop. I noticed the fluid was dark, so I decided to change it and bleed the brakes. I started by bleeding the rears. After several squirts, the pedal wouldn't pump up. I went to the front brakes and got nothing. no fliud can out and the pedal still wouldn't pump up. I went back to the rears and left the bleeders open, returning to the fronts. The pedal started to pump up now so I closed the rears and bled the fronts. Went back and bled the rears. Now I have a firm, solid pedal and haven't had trouble since.

Weird, huh?

mtman

plugged the vacuum line and tried brakes-- pedal did not feel hard and went down low--checked the reservoir and looked a little low even though i topped it of after yesterdays bleeding--also noticed the area around the back of the master cylinder where it bolts to the booster wet so i may have a leak--if i have to change the MC is it a big job--i have done most of the work on the jeep so far myself--thanks for the help

dave1123

I've never done a MC replacement on a Jeep, but I have on GM cars. The biggest problem I had was running all over the city to find adapters it fit the brake lines to the new one. Replacements don't seem to come with the right threads to fit the OEM brake lines. Why that is, I haven't a clue.

Otherwise, it's pretty easy. Just be sure you prefill and bleed it before installing.

pwb

its easy, i didnt had any problem with the line threads. I got a centric parts realy cheap at amazon i payed 45$ for it. The only thing i needed to do was to use the old reservoir since the reservoir that was on the new one was in level with the MC.
Use a lot of paper, the liquid isnt good for your paint.

3278

I have pretty similar symptoms, and as soon as my second pair of hands wakes up, I'm going to go try this diagnostic, but I also wondered: when I depress the pedal - every time - there's a loud chuffing noise, something you normally hear I think when the engine's off and you pump the brakes, but for me it happens every time. Does that sound booster-y or master-y to anybody?

3278

Okay, was able to do the diagnostic, and the pedal was very hard, and stopping the vehicle was very difficult. Does that mean probably the problem is the booster, then?

freegdr

is there vacuum to booster?

3278

Normally, yeah, but I did as ol"blue recommended above and pulled the vacuum line to intentionally take the booster out of commission. Hook the vacuum back up, and the assist returns, but the pedal is again spongy.

The previous owner strongly suspected the booster was bad, but he replaced it with the booster from a 95, which didn't have the right rod length, so the brakes didn't engage until you hit the floor.

freegdr

if you depress pedal keeping pressure on it does it hold or creep down ?

freegdr

rod length at pedal ?

3278

It's so spongy that it's hard to tell, but yeah, it seems like it holds.

That's correct. The rod on the 95 booster was the wrong length, so basically depressing the pedal 3 inches was like depressing the pedal 1 inch. No bueno. But we pulled it off, put a 98 booster in [the correct one for the rest of the braking system] and now the pedal stroke is correct, but the braking power is weak, and the pedal chuffs when you depress it like the seals in the booster are shot. I just wish I had a magic bullet test to tell if it's the booster or the master cylinder.

3278

Wait, now that I think about it, this doesn't make sense. If my booster is bad, it should act kind of like it does when I disconnect the booster: the pedal is super-firm, and hard-to-push. Instead, with the booster connected, the pedal is soft and spongy, as if there was a ton of air in the lines [but they've been thoroughly bled, as far as I can tell]. Does that seem like probably the master cylinder, then?

I guess if all else fails, I'll replace them both, but my nightmare is that I'll still have the same problem.

dave1123

Just a thought. Lift the vehicle and have someone hold the pedal down (with the engine off). Try to manually turn the tires. My problem was similar and doing this test we found the front brakes weren't working! After a complex bleeding, they worked fine. We assumed the shuttle valve that isolates the front brakes in case of a leak was sticking. We got it working by leaving ALL 4 BLEEDERS OPEN and pumping the pedal. They have worked fine since.