Brakes

[omega_rugal]

bad booster or vacuum leak, if turns out to be a bad booster replace it with a newer WJ dual diaphragm

 

in the mean time try to not drive the MJ like that, you gonna kill someone/yourself, MJs don't have the greatest brakes i could only imagine driving it without the vacuum assist

[gogmorgo]

I’m also pretty firmly on the side of the factory brakes being more than adequate, but only if you maintain them. I’ve successfully panic stopped an unbraked 3500lb trailer from 50mph behind my long bed with no drama. Incidentally that was also when I discovered uhaul dollies don’t have brakes. 
 

When I got my little MJ, the brakes were pretty mediocre. I flipped the brake proportioning linkage back the way it’s supposed to go (still not sure how it can overcentre on its own as I’ve had it at full droop and it didn’t come close), then bled the brakes properly, and now I’ve got no issues. FYI the brake prop valve bar is supposed to be horizontal, and the rod to the axle should be vertical. 

 

The test of a booster is to pump the brakes a few times with the engine off to relieve any residual vacuum, then push the pedal gently – as if you were just holding it in place at a light – and then start the engine. You should feel the pedal sink under your foot as vacuum builds just after starting the engine. If the pedal does not sink, make sure the booster is connected to vacuum before changing the booster. Make sure the hose runs from booster to intake, make sure all connections are tight, if the hose looks cracked, change it.

[AZJeff]

17 hours ago, gogmorgo said:

I’m also pretty firmly on the side of the factory brakes being more than adequate, but only if you maintain them. I’ve successfully panic stopped an unbraked 3500lb trailer from 50mph behind my long bed with no drama. Incidentally that was also when I discovered uhaul dollies don’t have brakes. 
 

When I got my little MJ, the brakes were pretty mediocre. I flipped the brake proportioning linkage back the way it’s supposed to go (still not sure how it can overcentre on its own as I’ve had it at full droop and it didn’t come close), then bled the brakes properly, and now I’ve got no issues. FYI the brake prop valve bar is supposed to be horizontal, and the rod to the axle should be vertical. 

 

The test of a booster is to pump the brakes a few times with the engine off to relieve any residual vacuum, then push the pedal gently – as if you were just holding it in place at a light – and then start the engine. You should feel the pedal sink under your foot as vacuum builds just after starting the engine. If the pedal does not sink, make sure the booster is connected to vacuum before changing the booster. Make sure the hose runs from booster to intake, make sure all connections are tight, if the hose looks cracked, change it.

I am well versed on booster function tests, and I am certain my booster was functioning as intended.  I remain convinced my old 1992 XJ brakes were marginal at best, and that the later booster is a significant upgrade.

 

Caveat—I have never driven an MJ with stock brakes, but I cannot think they would be massively different than an XJ of the same era.

[tugboat95]

Talk to me about the proportioning valve. If I'm going with a new master and booster may as well replace it. Are they available? Do you "clean" or rebuild them?. Can they be deleted?

89 Comanche
Eliminator
2wd
4.0L
5 speed PukeGoat
Factory Original

[JustEmptyEveryPocket]

4 minutes ago, tugboat95 said:

Talk to me about the proportioning valve. Queue Eagle: The MJ has a distribution block, not a proportioning valve. The rear brake valve (height sensing valve) biases the rear brakes based on bed height. But only if the rod is connected. Plenty of threads on this if you do a quick google search :site:comancheclub.com rear height valve". From here on, I am referring to this rear valve. If I'm going with a new master and booster may as well replace it. Only if you can find a NOS one, or a used one in better condition than yours.  Are they available? No. Do you "clean" or rebuild them?.  Never seen it. Not saying it can't be done though. Can they be deleted? Yes. Several ways to do it. Again, lots of threads, so just google search. Something like "site:comancheclub.com delete rear height valve". 

 

Good Luck!

 

[gogmorgo]

Booster test info was for the OP. Lots of talk about replacing it, not much talk about actual diagnosis.

 

Obviously there will always be room for improvement in any braking system. My experience so far however is that 30-year-old vehicles are frequently neglected, and thus it’s often difficult to establish a proper baseline for comparison. And yes, if your metric for comparison is a newer, more modern vehicle, then even an MJ in factory original condition may seem less competent, especially if you’re making the brakes work harder with bigger, heavier tires like most of us do.

 

And yes, parts do wear out, and if there’s room for an upgrade that takes a similar amount of work and doesn’t cost much more than repairing the problem, by all means do so. I don’t really want to seem like I’m advocating not for modifying anything. The point I was trying to make was that the brake system, in its original configuration, does in fact work quite well. If it’s not doing that, there’s a problem. Throwing new upgrades at certain areas without knowing what the problem was only fixes the problem if you’re lucky and inadvertently solve the problem. If you don’t get lucky, the upgrade might help a little, but mostly it just means you did a lot of work for not much result.

 

17 minutes ago, tugboat95 said:

Talk to me about the proportioning valve. If I'm going with a new master and booster may as well replace it. Are they available? Do you "clean" or rebuild them?. Can they be deleted?

[Eagle]

9 hours ago, tugboat95 said:

Talk to me about the proportioning valve. If I'm going with a new master and booster may as well replace it. Are they available? Do you "clean" or rebuild them?. Can they be deleted?

89 Comanche
Eliminator
2wd
4.0L
5 speed PukeGoat
Factory Original
 

 

You're new here, so a little background. The XJ (Cherokee) has a distribution block in front, below the master cylinder, that also functions as a proportioning valve. The MJ has a similar looking distribution block under the master cylinder, but it is NOT a proportioning valve. The proportioning in the MJ is handled by the load/height sensing valve mounted to the left side of the frame, above the rear axle.

 

 

 

Where things get more complicated is that the MJ system has two hard lines from the front to the back. One of them, the one that comes out of the "nose" of the distribution block, is for normal operation. It feeds brake fluid into the rear height sensing valve, so the amount of rear braking is proportional to the amount of load in the bed. But, if the front brakes fail, that would leave you with reduced (proportioned) braking at the rear wheels. So they built in a bypass circuit that opens up only if the front brakes fail, and bypasses the height sensing valve to send full pressure to the rear brakes.

 

No, the rear height sensing valve is not available. Yes, it can be bypassed, and you can find instructions on doing that on this forum. Here's why you may not want to do that:

 

Why do they proportion the braking to the rear? Because a pickup truck, when empty, has very little weight over the rear axle and wheels. That means the rear brakes would lock up long before the front brakes, especially when the bed is empty. Premature lockup of the rear brakes causes spin-outs. So they proportion the rear braking as a safety feature, to reduce the likelihood of spin-outs.

 

I had to delete the rear height sensing valve on one of mine, because it exploded in a panic stop situation. So I did the bypass/elimination. Guess what? My rear brakes lock up early. In my case, that's not too bad because I'm old enough that I grew up and started driving long before there were such things as proportioning valves. I got my driver's license in 1960. The first vehicle I bought with a proportioning valve was my '88 Cherokee. By that time I had 28 years and almost a million miles of driving behind me, so my right foot expects the rear brakes to lock up. I can deal with it. You younger guys, who have probably never had to deal with that, probably should think seriously about intentionally creating a situation that invites premature rear wheel lock-up. It's better to repair the brakes and keep them operating the way they were designed.

 

 

[tugboat95]

So I finally got the time to do my brakes. Pretty much replaced everything on the front including the bearings. Everything new. Next time home I'm going to have somebody do the rear. I hate doing drums. Worth it to me to pay somebody else. Doing this so I know whats there. Mainly worried about wheel cylinders. (Past experience and a wreck that cost me some money) But if it comes apart I'm putting all new stuff back.

As to why my brakes were horrible? I don't know. I got brakes now!! I have plenty of pedal, and I can lock them up. I think somebody rebuilt the calipers instead of replacing. It had new shiny fittings and such on them. Maybe something off with one of the pistons???? I checked the booster and it appears to be working. It's getting vacuum and the line is good. It does have some corrosion on the bottom but I think I'm going to leave it for now.

89 Comanche
Eliminator
2wd
4.0L
5 speed PukeGoat
Factory Original