F1Addict

On my way to REI I went to stop at a red light and my brake pedal just went to the floor. I quickly yanked the parking brake with just 20 feet before hitting the car in front of me. My rear tires locked up and I came to a screeching halt, a little bit sideways and a lot panicked but before I rear ended the ML63 stopped at the light ahead of me.

I limped home using my parking brake (which worked fine). First thing I did was inspect the wheel cylinders for signs of leaking. Calipers and drums are completely dry. Then I checked the brake fluid level in the master cylinder reservoir and it was at the correct level. Next up, I pulled the vacuum line to the booster and it's producing excellent suction, there are no signs of vacuum leaks. I then tried pumping the brakes with the engine off and it appeared most of the resistance was being produced by the spring, not hydraulic fluid. This is a California Jeep so zero rust, soft lines were replaced two years ago, calipers were rebuilt and wheel cylinders replaced. Shoes and pads were replaced at the same time and still show 60-70% of pad material with even wear.

I'm pretty confident this is being caused by a failed master cylinder. I just replaced the master cylinder in February because of a creeping pedal. Since then my brakes had worked perfectly. Well, up until this afternoon.

Am I on the right track or could it be something else? Proportioning valve? Brake booster? Something else?

F1Addict

Alright, I pulled the master cylinder and inspected the seals/springs on both the primary and secondary pistons. They appear normal and undamaged so I bench bled it and reinstalled. Now I'm thinking the seals on the booster might have failed. I have noticed that depressing the brake pedal while at idle causes a ~500 RPM increase which would indicate a vacuum leak when the pedal is depressed.

cruiser54

Perfect time to upgrade to a WJ booster/master......

But, it sounds like a master cylinder failure to me.

F1Addict

Thanks Cruiser. Sadly, there are no WJs in my local junkyard so I'm kind of stuck since this is my DD and it's going on a 1,000 mile road trip next Monday.

I wish there were a straight forward way of testing the booster and master, individually, rather than just blowing cash on parts until it stops not working. I'm going to pull the booster and see if there's anything obviously wrong with it.

cruiser54

Booster gives a hard pedal and master gives a "go away" pedal.

Amy 95 to 96 XJs in the boneyard?

DFlintstone

If there are no leaks and the res is up, I can only think of two things. If the front had something like a wobbly rotor or bad wheel bearing, a piston or piston's in the caliper could be back to where the first stab gave nothing. Follow? BUT the prop valve should still give pressure to the rears. (the springs there should assure the slaves are home. Grasping here, but covering bases. If the prop valve had tripped before, and you were running on just fronts, then had some play in a rotor to push a piston back, that could do that. In this case, it would pump back up pretty easily.

A failed master is way more likely. Both my 90's, one Bendix 9 with ABS, and my other both will work without the booster, petal is just way harder.

OH MMM #3. Wonder if a flex line could "balloon", let the pedal down but not leak. I really don't know there. Repeated, fast hard pumping might burst it if that were the case. Goofy idea I suppose.

F1Addict

Just rechecked and the page updated. Lots of Jeeps now.


92,95,96,98,01 XJ
(2)00,(3)01,02,(2)03 WJ
93,(3)96,98 ZJ

I pulled the booster but I have no idea what I'm looking for. The check valve seems fine, only flows one direction. Lots of decomposed foam around the input shaft (pedal side). The pedal doesn't have a go-away feeling, it just goes to the floor with little more than mechanical (spring) resistance. As I said, the master is just three months old, replaced due to a sinking pedal. It seems ridiculous to me that it's failed so comprehensively after just three months when it replaced the one that had been in there for the last 27 years but I have to accept the possibility.

cruiser54

Welcome to the new real world of aftermarket replacement parts.

Warranty it.

F1Addict

Thanks Don and Cruiser. I replaced the soft lines a couple years ago when I overhauled the whole works (rebuilt calipers, replaced drum cylinders, drums, rotors, pads, shoes, springs, rebuilt OEM master cylinder and new soft lines). My rebuild didn't last long because it wasn't the seals that were failing, there were pits in the cylinder due to moisture contamination (I assume) that was allowing fluid to seep past the seals. I replaced the master back in February but it is a cast iron import. I couldn't find an OEM.

I'm going to warranty the master cylinder. I just read through the WJ booster swap write-up. It looks like an awesome upgrade but it's a lot more complicated than anything I want to tackle right now. I need reliable and I need it quickly, I'll upgrade later.

Hopefully the master cylinder takes care of it.

F1Addict

They replaced the master cylinder under warranty, that fixed it. Same crappy Chinese master cylinder so I guess I can plan on replacing it again in a couple months... hopefully with one from a WJ, along with the booster.

Thanks for the help.