MJ Comanche Tech. Stock & Modified. MJ (86-92)
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Can't Stop My Truck!

Old 10-17-2016, 08:08 PM
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Default Can't Stop My Truck!

I have an 87 Comanche that is hard to stop. I can literally stand on the brake pedal with both feet and the truck comes to a slow stop. Can't even lock up the front wheels. Have replaced: front pads, rear shoes, bled and bled the system, replaced front calipers, and replaced the vacuum check valve at the power brake booster (have lots of vacuum). Stumped at what to do next. If the problem is the master cylinder or brake booster, how do I check these? Thanks!
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Old 10-17-2016, 08:16 PM
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if you didn't do anything to the master, it's probably fine. booster is bad if it is really hard to push the brake pedal.

i think your problem is the calipers. you most likely installed them on the wrong sides.
check to see if the bleeder is above the brake line.

you could also swap out the booster to a dual diaphragm from a newer 97 and up xj. pretty much bolt in, other than reaming out the brake switch hole and grinding the end flat for your old style switch.
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Old 10-17-2016, 08:18 PM
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U also could do a WJ booster & master cylinder swap Heard it's much easier than using 97+ XJ booster
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Old 10-17-2016, 08:44 PM
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How does your rear load sensing valve look? If it's not installed correctly you can have brand new brakes and it won't make a difference...
And yes the wj booster and master cylinder is THE best upgrade I've done, the difference is night and day! And not hard to do just make sure you have the tools to do it...
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Old 10-18-2016, 06:21 AM
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Thanks all for the replies. As for the installation of the calipers, I believe they are correct but I will double-check.
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Old 11-07-2016, 12:25 PM
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Try this:

http://comancheclub.com/topic/27326-...nsoring-valve/

Had the same problem with my '88 then I did this. It helped a little.
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Old 11-08-2016, 04:57 PM
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Did you bleed the the brakes correctly? MJ's have a load-sensing proportion valve under the bed, tied to the rear differential. It has a bypass line going to it in case of a panic stop. Because of this, there is a whole 20 step process on how to properly bleed your brake system. It's not like that of an XJ. If air has gotten in the system, and I suspect it has, you will definitely need to bleed them correctly to regain braking.

You can also convert the whole truck to XJ style brakes. That will require you to replace the MJ distribution block with an XJ prop valve block, remove the load-sensing valve (and bypass it), and then you can bleed the brakes like any other non-ABS system. I did that on both my MJ's.
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Old 11-09-2016, 09:47 AM
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Dana35? You could also convert from drum to disc.
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Old 12-06-2016, 05:27 PM
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My 90 XJ had the worse brakes of any vehicle I have driven when i bought it, and I've driven older vehicles. Wouldn't lock the wheels up with 31s on it and all new brakes. Swapped to the WJ booster with ZJ prop valve guts. Will lock up my 33s now, and stops good. Easy enough swap. Biggest issue was flaring the lines from the booster to the prop valve. Wasn't hard, just had to borrow the flaring tool from my neighbor.
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