MJ brakes not working

I have just finished my 4.0 H.O in to my 1988 comanche and had to replace a few rear brake lines. I Have bled them several times but they still don't work right.I can't even drag them on the wet dirt. I think it may either be the booster or master cylinder. i have to pump them several times with the engine running to get much resistance. i don't think the proportioning valve is connected. the front brakes work but not like i would like. please help me stop this jalopy.

Not sure what your problem could be exactly. However, I know MJ's have a load sensing proportioning valve in the rear. My dad's dakota had a similar set-up and caused some problems with his rear brakes, you might want to look into that.

Yeah, the Comanche is supposed to have a load sensing proportioning valve. If it hasn't been removed from your truck, it'll be located just above the rear differential, and there should be a link (a rod about 1/4" dia) from the diff up to the arm on the valve.

I haven't disassembled one of the valves to see what makes it tick, but I know there's a return line from the valve back to the combination valve in the front (which on the MJ apparently doesn't function as a combination valve). I don't remember if there's a special procedure to bleed the rears on the MJ, but first check to see if you have that valve. If so, let us know and I'll look up the procedure.

It should be relatively easy to track down the problem. While someone's holding the pedal, loosen the fitting off the m/c going to the rear brakes. Fluid comes out? M/c's good. Keep doing this as you go to the rear of the vehicle and you'll find the problem.

simple solution:

buy combo valve for XJ, put in MJ, remove and discard clusterfawk in rear.

I wasted about 18 bottles of brake fluid trying to get the damn brakes bled in my MJ w/ that stupid valve in the back. ripped all that sh1t out and NEVER had a problem since.

put lotsa miles on it like that, hauled WAAAAAY more than it should have and never had an issue w/ rears locking up prematurely.

was yours a 4-cyl? mine was. this was immediately after my 4.0L swap.

4.0/auto & the 4.10s w/ 235s was awesome. kinda miss that truck being set up like that.

I have the valve on the back but it appears to be a block of rust and is disconnected. there are two lines going back from the valve in the front and they just T together with the line on the axle. I think the fluid is coming from the master cylinder and going right back. If I pump the brakes a few times then there is resistance but if i stop it goes away. I think I will try the XJ proportioning valve see if that works.

cncman said:

I have the valve on the back but it appears to be a block of rust and is disconnected. there are two lines going back from the valve in the front and they just T together with the line on the axle. I think the fluid is coming from the master cylinder and going right back. If I pump the brakes a few times then there is resistance but if i stop it goes away. I think I will try the XJ proportioning valve see if that works.

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It will work, but I wouldn't bother.

On my '88 MJ shortbed, I popped a rusted rear hard line when I was cut off and had to do a panic stop. All 4 wheels locked up, but it burst a line right over the gas tank. I replaced the line at a friend's garage so he could help me bleed the brakes when we finished. Got the new line in, went to bleed, and my friend got a face full of brake juice. The proportioning valve had also burst. Didn't have access to a new one and didn't have access to an XJ combo valve, so we just cut off and plugged the return line and by-passed the rear proportioning valve. Lawn Cher' has done the same thing on his MJ (which is a longbed).

It appears that the MJ combo valve isn't a combo valve -- I don't think it includes a proportioning function, only the brake system failure light function. Other than that it's just a junction block, which means you get full braking to the rear at all times. I know that pickups are light in the rear when not loaded and that this can result in early lockup, but I have had a problem with the rears on mine locking up exactly once. It was in late winter, when there was still sand on the roads. It was a rainy day and I was approaching a 4-way stop sign going down a STEEP incline. Under those conditions, most of the weight moved to the front wheels and, combined with the rain and the sand, the rears did lock up and I skidded a bit sideways.

Other than that one incident which combined about all the problems that would contribute to rear wheel lockup, I have never noticed any problems. So I think you could just remove the return line. It's the line that goes into the front end -- the "nose" -- of the combo valve. Plug that outlet with a standard bolt of the correct thread, connect the other rear line directly to the flex hose from the axle, and you should be good to go.

If you find that you're consistently getting the rear brakes locking prematurely, you can always go back later and swap in an XJ combo valve. The physical size and shape are identical -- the XJ valve simply doesn't include the return line coming off the "nose."