I have terrible brakes, but why

88 MJ; but should be teh same as XJ.

When I first got it, the brakes were nothing to write home about. Marginal, at best.

The pedal wouldn't do anything until halfway to the floor then they'd feel all spongy and crap. So I just bled the whole system, front and rear. The fluid that was in there was dark and nasty; but full. The pads/rotors, shoes/drums are all in good condition with plenty of meat left.

When I hit the brakes, the pedal moves very far down without any feeling of resistance, then about 2/3 the way down, I can feel the brakes start to engage. I can hold the pedal to the floor, but the truck just slows down quickly; it doesn't actually stop. Sitting still at a light, the pedal is probably 1" off of the floor and that is just barely enough to keep the truck from taking off.

There is quite a bit of feedback in the pedal. I can feel the pedal surging back under my foot rhythmically; the speed of the surge is dependent on how fast i'm going. Slow speed, slower surge.

I'm wondering that since the pedal travels over halfway down before there's any engagement of the brakes what it could be. I know what spongy brakes feel like, and the first half-ish part of the pedal stroke there's not even that. There's no visible brake fluid leaks anywhere and the lines seem to be in good condition.

Any ideas?

My '88 MJ has terrible brakes too. It is definitely better now that I have done away with the load sensing valve on the rear axle. Do a search at
comancheclub.com

Did you have a similar problem before you removed the LSV?

I hope you are not driving it like that. You have numerous problems. Brake pulsing varying on vehicle speed is rotor thickness variation and possible rotor runout or rear drum out of round. Bad unit bearing(s) in the front can also cause the rotors to runout excessively. This can push the pistons away from the rotor too far for the master cylinder to push back when you press the pedal. Rear brake adjustment can also have this same effect. You also need to get all the nasty break fluid out of the system. Do a complete fluid change. But the pulsing is definately a mechanical issue that needs to be repaired. Jack up the front and check the wheel bearings for free play. If that is good check the rotor thickness for variation with a micrometer. FSM says no more than .0005" variation and .005" runout with an indicator. If all this is good, the rear drums need to be checked for roundness, which is more difficult.

Once you find out what is causing the pulsing, you can go on the tackle the travel and spongy issue. But bad wheel bearings could cause almost all of the issues.

My MJ had terrible brakes too. I think it's the single-diaphragm booster. I swapped in a booster and master from a 99 WJ and it is 100x better. And I also have Explorer rear discs.

I disconnected my LSV too and just zip-tied it out of the way. I have a prop valve from a 4-disc ZJ that's waiting it be installed. Then I'll remove that LSV and its extra lines once and for all.

No, I'm not driving it like that since it's a safety hazard. Though, I did drive it to the dump because our trash was piling up.

The rotors are still very thick as are the pads.
I performed a full fluid flush.

I'm mainly concerned with the travel issue. When the pedal gets most of teh way towards the floor, I can feel the brakes grab.

As far as the pulsing, it feels a little different than the usual feeling you get from warped rotors; it's hard to explain.

I'll check the wheel bearings.

How would bad wheel bearings cause the brake pedal to travel so much?

The wheel bearings having too much play, would let the rotor flop around with respect to the calipers, which pushes the pads away from the rotors. Then the master cylinder stroke can't displace enough fluid to move the pads all the way back in. You should be able to pump it up to get a better pedal and move the pads back with a few master cyl strokes. Then, even if you sit a while, the pedal should be good, until you drive it a little, letting the rotors push the pads back away too far. If it has to be pumped up again, even if you don't move the vehicle, I'd suspect the rear drum break adjustment. But the rear drum adjustment would not cause pulsing.

I had the same problem on my 93 XJ, found that the brake booster was bad, try disconnecting the vacuum line from it and see if it makes a difference in the pedal.

Will it make the pedal stiff and hard to push?

bigalpha said:

Will it make the pedal stiff and hard to push?

Click to expand...

Removing the brake booster vacuum line should make the peddle harder to push.

Part of your problem sounds like bad master cylinder. I had similar problems for years even after replacing the MCs with rebuilts, and I finally got fed up. I went to brand new (not rebuilt) cast iron only (not alluminum) master cylinders on mine, with new vacuum brake boosters. All the rest of my brake stuff was new, except for the Prop valves.

The rebuilt MCs seem to leak out the rear and take out the vacuum diaphragms.

I recently bought a brand new brake drum from AZ that was way out of round! It was so bad, you could visually see it on the inside.

If you pump up your pedal and hold it down firmly, it should not continue on down to the floor. If it does, you have a leak. It could be wheel cylinder to the outside, brake line to the outside, or master cylinder to the outside or even just inside. If the pedal holds after you pump it up, you don't have a leak. (At least not a significant one.)

Thanks for the advice. I'll give it a shot when I get home.

I'll pump the brakes and see if they build pressure and hold.
When I pulled the rear passenger tire, there was a lot of fibrous looking crap all built up inside the drum; but the shoes/drums are in good shape.

I'll pull that tire again and take a closer look. The first time I pulled it, it was like 11pm.

The rear brakes also need to be adjusted out with the adjuster wheel (I call it the "star" bolt), until they just rub VERY SLIGHTLY. All the adjuster mechanism can use regular disassembly and lubing for the self adjusters to work properly.

Adjusting rear brakes: They are supposed to be self adjusting, but that shit freezes up quite a bit, and even after I fixed mine, it seemed like the original self adjusting cable was made too long to adjust properly. The best is to take all this stuff apart (pull the drums off and lube it all up including dia-assembling the adjuster star wheel). But, thru the slots, you can tighten up the free play in the rear brakes which could be taking up more fluid than the stroke of the master cyl can displace. If you never did it before, you probably need to pop off the rear drums and look at everything.

Alright, I'll double-check the adjustment of the drums, too. I pulled the drums once to bleed the brakes, but it was pretty dark out so I didn't get a chance to really see things very well.

I replaced the adjusters on mine, and when I did I discovered that the old adjusters points or tips were worn off to nubs compared to the new ones. They made a big difference. Also be sure to lube the threads on the adjusters with a good high temp thread antisieze compound. You might want to check the drums and rotors for excess wear, beyond spec too.

Alright, pumped the brakes but it did not change. When I push the pedal, I get a very loud hiss of air. The XJ does not make that noise. I'm leaning towards bad MC.

Pulled the drum and cleaned that side; there was some fibrous looking crap on it. The drums on both sides are adjusted properly; there is a slight contact when the tires are rotated.

Forgot to pull the vacuum hose on the booster, but it's raining now. Sadness.

Load hiss is a bad Vacuum booster. Sounds like MC and Vac booster are both bad. Like I said, my rebuilt MCs leaked out the rear and took out the vacuum booster diaphragm, on two jeeps. :gonnablow

Today, the jeep I have with the best brakes went in for inspection. Found out during the dyno test that only the front brakes are, were working (the rear was running on the dyno and the tester stood on the brakes and they would not grab on the rear!) he had to use the emergency brake lever. The front cell of MC (feeds the rear brakes) was a little low, and I had just filled it and bleed it 2 weeks after replacing everything brake wise on the drivers rear side when I replaced the bent axle from the fender bender. :dunno:

But it has the best brakes of the three Renix jeeps I have (except maybe the one I finshed 4 weeks ago with the brand new MC and VB)?

Are the booster and MC pretty easy to change out? I imagine it involves another bleeding of the lines to clear any air leftover.

Parts of the task are a pain, like standing on your head under the dash, legs tuck up your $ss, to get the four nuts off the back side of Vac BB, and to get the brake peddle stuff, and brake switch parts loose of the Vac BB shaft. Being able to immulate a pretzel with your arms and hands helps while under the dash.

Actually I had the hardest time replacing the metal lines from the Prop valve to the MC. I gave up trying to pull and service the prop valve, as the lines have been on there for 25 years now, and they ain't coming loose unless I nuke them. :gonnablow

But all in all, it is not that bad. Could be done in 2-3 hours by a pro, Took me 3 weekends, fighting rain, heat, and waisted 2 of those week ends trying to get the Prop valve and its lines loose, with out damaging them. If I was prepared, with all the parts, nice weather, etc, it's about a 3-4 hour job for me, but I am getting old, and move slow. Takes another hour to bleed everything (other than the mechanic that is, LOL).