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Soft Brakes


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Hey guys, I finally picked up a MJ after looking for one for about 5 years. 1988 4.0 auto 4x4 long bed. 

Anyways, the brakes are real soft. I’ve bleed them, verified the booster has good vacuum, and it has a newer master. PO just replaced the drums/pads and rotors/pads. 

 

I’ve read on here and the cherokeeforum but I’m coming up short. The pedal moves a good 2-3” before brakes engage and I have to put it to the floor to just come to a complete stop at stop signs. 

 

Any other suggestions I'm missing? Or are these trucks just that weak in the brakes? I’ll end up doing the ‘96 xj booster swap but no one has them in stock right now and none at the jy either. 

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Does it still have the rear height-sensing (load-sensing) proportioning valve intact? If so, did you follow the prescribed procedure for bleeding Comanche brakes? Hint: It's not the same as for the Cherokee, so I don't know what you learned from the Cherokee forum.

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4 hours ago, Tamadrummer said:

Also could it be the vacuum actuator on the front axle? The soft lines on that just fall right off. If I’m correct my Xj didn’t have that even though it was a 91. 

 

 

 

Doesn't affect the brakes. If you're losing vacuum through the center disconnect fittings, you'll have a hard peddle (feels like no power brakes, because you'll have no power brakes). It won't cause a low peddle.

 

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1 hour ago, Tamadrummer said:

Would not being able to bleed the bypass cause them to be so soft? If there is residual air in the bypass?

 

Yes, that's why there's a special procedure to bleed the bypass.

 

Also, if you're not the original owner, make certain that the calipers aren't on the wrong sides, which would put the bleeders on the bottom rather than the top.

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Have you verified that the rear drums are actually working? If the shoes are not properly adjusted, the pedal will feel like crap no matter how well the system is bled. I've worked on many, many rear-drum-brake vehicles, my MJs included, that had no other problems except the rear brakes being out of adjustment. Just last month, I worked on an acquaintance's Saturn with a very, very similar complaint. It turned out to have a broken adjuster cable in one of the rear brakes.

 

Assuming the PO actually put the drums back together correctly (and this is far from certain), find somewhere where you won't bother anyone and do some alternating forward and reverse stops and see if the pedal starts feeling any better. You can also try working the parking brake. If that makes the pedal feel better suddenly, the problem definitely lies in the rear drum brakes.

 

What went wrong when you tried to bleed the bypass?

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I have not verified the rear drums are functioning yet. But when I followed the bleeding process like the picture I posted the open front bleeder was spraying fluid which makes me think the bypass didn’t engage, correct?

 

I had a spare tranny and t-case in the rear but guess it’s not heavy enough to increase the rear brake pressure. 

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8 hours ago, Tamadrummer said:

I have not verified the rear drums are functioning yet. But when I followed the bleeding process like the picture I posted the open front bleeder was spraying fluid which makes me think the bypass didn’t engage, correct?

 

 

Incorrect. The bypass function is to allow full (unproportioned) brake pressure to the rear brake circuit if the front circuit fails. The purpose of opening a front bleeder is to simulate a loss of pressure in the front brake circuit. Obviously, to do that the front circuit has to spit out the fluid through that open bleeder. Seeing fluid come out the open bleeder in no way indicates that the bypass isn't working.

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8 hours ago, Tamadrummer said:

I’m new to that differential valve, but when it’s engaged in 4x4 it wouldn’t change braking power distribution?

 

The rear height- (load-)sensing valve is not engaged or disengaged with 4WD. It is what the name implies -- it's a valve that opens and closes based on the height of the chassis relative to the rear axle. The valve is located on the frame above the left side of the rear differential. It is connected to the differential (or should be) by a vertical rod. When you put load in the bed, the bed settles, which pushes down on the rod. The rod can't move, so the lever arm on the valve is raised, allowing more brake pressure/flow to the rear brakes.

 

It works exactly the same in 2WD and 4WD.

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On 11/22/2018 at 10:15 AM, Eagle said:

 

The rear height- (load-)sensing valve is not engaged or disengaged with 4WD. It is what the name implies -- it's a valve that opens and closes based on the height of the chassis relative to the rear axle. The valve is located on the frame above the left side of the rear differential. It is connected to the differential (or should be) by a vertical rod. When you put load in the bed, the bed settles, which pushes down on the rod. The rod can't move, so the lever arm on the valve is raised, allowing more brake pressure/flow to the rear brakes.

 

It works exactly the same in 2WD and 4WD.

Right, I was referencing the vacuum actuator on the front axle for the 2 piece axle shaft. It’s vacuum engaged and was wondering if it would affect the vacuum for the brake booster. I don’t believe it matters for the brakes as one line was off the actuator and placing it back on did not change anything. 

 

For the bypass, when I tried to bleed the rear with the front open, the front was spraying/leaking too much fluid and it about drained the MC. Since it let that much fluid though the front I’m assuming the bypass is not working as it should. I’m new to the bypass but if it was functioning properly I would guess less fluid would be distributed to the front when the bleeder is left open. 

 

Should I do the delete/bypass on the rear height load sensing valve? The truck looks like stock height on 31’s but I will eventually lift it

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40 minutes ago, Tamadrummer said:

Right, I was referencing the vacuum actuator on the front axle for the 2 piece axle shaft. It’s vacuum engaged and was wondering if it would affect the vacuum for the brake booster. I don’t believe it matters for the brakes as one line was off the actuator and placing it back on did not change anything. 

 

 

If the booster doesn't have vacuum, you get a high, hard peddle, not a low, soft peddle.

 

Quote

For the bypass, when I tried to bleed the rear with the front open, the front was spraying/leaking too much fluid and it about drained the MC. Since it let that much fluid though the front I’m assuming the bypass is not working as it should. I’m new to the bypass but if it was functioning properly I would guess less fluid would be distributed to the front when the bleeder is left open. 

Should I do the delete/bypass on the rear height load sensing valve? The truck looks like stock height on 31’s but I will eventually lift it

 

 

The bypass bleed procedure has to drain the FRONT brake circuit to open the bypass. The front and rear chambers of the master cylinder aren't connected, so draining the reservoir for the front circuit doesn't affect the rear circuit.

 

Quote

I’m new to the bypass but if it was functioning properly I would guess less fluid would be distributed to the front when the bleeder is left open.

 

 

No.

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Go at it again and let the front brake fluid drain completely? Then follow the instructions above? I didn’t even put 2 and 2 together about the different brake resivours :doh: I’ll try that but still want to eventually do the brake booster upgrade. Thanks for all the help so far, going to be a lot of this going on lol 

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