strange brake behavior

I'm no Brake Wizard, but the only time my Brakes ever went to the floor is when my Master Cylinder piston let hydraulic fluid bypass the seals i.e I had a bad Master Cyclinder (rebuilt). Many (I'm sure not all) rebuilt master cylinders are not perfectly honed and polished. I chucked my rebuilt and bought a new Master Cylinder and the thing works perfect.

Are you using a rebuilt master cylinder? This might be the problem

It's hard to believe, but the Jeep could have been at an angle extreme enough for the master cylinder to suck air. That's scary. The part about the brakes being applied even though you had your foot off the pedal is strange though. I would probably replace the master or at least tear it down for inspection just in case.

Earle

What's really strange is that no fluid is missing and the brakes did not require bleeding. My best guess so far after looking at the FSM is that the atmospheric pressure valve could be leaking. This would mean that even with the foot off the pedal there is still no vaccuum at the backside of the diafragm, causing the brakes to stay applied by the booster itself via the secondary pushrod. The brake pedal could come back up moving only the primary pushrod. What would you recommend, change the booster or the entire master cylinder and booster assembly? I'm leaning towards replacing the booster only for now.

As far as I know it is the original master cylinder. Thanks

What's really strange is that no fluid is missing and the brakes did not require bleeding. My best guess so far after looking at the FSM is that the atmospheric pressure valve could be leaking. This would mean that even with the foot off the pedal there is still no vaccuum at the backside of the diafragm, causing the brakes to stay applied by the booster itself via the secondary pushrod. The brake pedal could come back up moving only the primary pushrod. What would you recommend, change the booster or the entire master cylinder and booster assembly? I'm leaning towards replacing the booster only for now.

nrs, if the Master Cylinder piston has bad seals, it will allow the brake pedal to go all the way to the floor and you wont have any loss of fluid. The fluid still stays in the brake system, but it's not being applied to the calipers. I'm betting on a bad Master Cylinder.

But I'm no Brake Wizard. Take my advice with a grain of salt. Have a Brake expert check it out.

The part that gets me is the intermittent part. However, if it is indeed the master cylinder it will eventually go all the way to the floor and stay there.

I have had booster failures before and "usually" it makes the pedal get real hard.

Earle

..

Ok, I'm pretty much convinced it's the Master Cylinder. I'll have it checked by a brake expert. Thanks.

All my driveway testing (following the FSM instructions) comes out just perfect. Anybody who tests it now will conclude I imagined the whole incident and wonder what I was smoking!

I had a brake booster that did that once. Worked fine in the driveway, but enter a parking lot with a lot of Mercedes and Audis in it, the pedal got rock hard and you had to stand on it just to slow down. After a couple of close calls I got a new booster.

Earle

Funny, I have a similar reaction at the beach when I'm around 20 somethin's in bikinis. But I don't think I need a new booster.

Spdloader

All it takes is a sticky check valve. The original problem seems to have occurred in a really non level position while running at idle.

Thanks to everyone for your replies. I'll post the resolution once it gets sorted out.

I had it checked out by a trusted shop and they found that the master cylinder was leaking a bit internally. It was replaced and seems to be working fine. I'll try to test it at extreme angles soon!

Here is a link to a picture of the extreme angles I'm talking about: