Sudden Brake fade???

I’m driving to work the yesterday and my brakes are feeling a bit soft. Not like the jeep wont stop but the pedal feels like it has a lot of free play. The problem is getting slowly worse. I can push the peddle to the floor. I recently replaced the front brake soft lines. So at first I’m thinking I must have a leak but I check the reservoir and the levels are fine. No sign of a leak. If I’m not losing fluid what is causing the problem?

Bronco said:

I’m driving to work the yesterday and my brakes are feeling a bit soft. Not like the jeep wont stop but the pedal feels like it has a lot of free play. The problem is getting slowly worse. I can push the peddle to the floor. I recently replaced the front brake soft lines. So at first I’m thinking I must have a leak but I check the reservoir and the levels are fine. No sign of a leak. If I’m not losing fluid what is causing the problem?

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If there's really no leak, it's pretty much gotta be the master cylinder.

I’ve never had a bad MC what do they feel like? Do they tend to go all at once like this? Am I going to have no brakes at all in three days?

Bronco said:

I’ve never had a bad MC what do they feel like?

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Pretty close to what you're describing. Increased pedal effort, a lack of stopping ability.

Do they tend to go all at once like this? Am I going to have no brakes at all in three days?

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In my experience on three other cars (but not the XJ), yes. You might be OK for a few days, or you might not; it depends on how bad the fluid loss is. It's not something that there's an absolute answer to. Either way, the brakes aren't working so you need to check it out.

Check for leaks around the master cylinder. Note that fluid could be leaking back into the booster (in which case you won't necessarily see it externally). Unbolt the MC from the booster and look inside of the booster; if you can see/touch brake fluid, the seals or piston on the MC are most likely shot. Also examine the diaphragm inside the booster with a flashlight for signs of tears; if there are any, brake fluid may be ending up in the booster itself and you can't see it. Tears on the diaphragm but no indication of fluid loss usually mean that the booster diapraghm needs to be replaced. Be sure to align the actuating arm with the piston on the MC when you reassemble it or you may end up with no brakes at all.

Having said that, also inspect all hard and soft lines for evidence of seepage. Look for dark spots where fluid has leaked out, particularly around joins. Remember to also detach lines (where possible) and look on top of them for cracks.

Quite honestly, while you can rebuild an MC, you're probably better off replacing it with a new one - if the piston has worn down, new seals won't seat around it properly and the fluid level will keep dropping.

Your call - check it out and see what you think, but there's a good chance it may need to be replaced.

I've had a couple of master cylinders go bad without any sign of external leakage. When the pistons inside wear, fluid starts to leak past them, and you lose braking power as the pedal sinks slowly to the floor. Eventually, if you neglect it, there will come a day when the the pedal just lets go and drops, no brake at all. Sometimes it will make a difference how quickly you hit the brake, and it will work better when you do it fast, but not when you do it slowly. The rubber cups in the master cylinder are designed to expand outward under pressure, so the harder you push, the harder they expand and the less they'll leak. If you've ever tried to get the last few chuffs out of a worn-out bicycle pump, you'll recognize the symptom.

So to test for that kind of MC failure is really pretty easy. After exhaustively checking for leakage or seepage anywhere else, just start the engine, push slowly and firmly on the brake, and if the pedal gradually sinks, that's it.

Ok this is good info, but both reservoirs look totally full. My logic is if there is not fluid missing there is no leak, internal or external. The other thing is that the peddle feel is not

casm said:

Increased pedal effort

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It's quite the opposite. It may feel like the seals are not catching. But once it catches it holds. It dose not continue to droop if I hold pressure on it. It passes the booster test. If you pump out all the vac with the Engine off and press on the stiff peddle, then start the engine. The peddle falls to its normal spot. Dose this still sound like the MC. That would make sense I guess.

Mark, when is the last time you adjusted your rear brakes. I know my D44 adjusters suck, and when i readjust them, It feels like I have new brakes. It's at least worth check out.

Bronco said:

I didn’t even know they where adjustable! So it’s been a while. I think I need to re build them anyway but that wouldn’t explain the sudden problem. That would develop over time. My brakes have always kinda sucked on this thing.

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I thought the same thing, and then I adjusted them. I did have one drum explode on me about 1 month ago. Ruined a perfectly good trip to Coyote. Always rember to cary spare parts!