tcase locks rear brakes on commanche

I am working on a buddies commanche. the factory axles were shot.

We set a pair of new axles under it. using the d30 front and rear 8.25 from a 2000 xj. the axles swap is simple and easy. A little grinding flip the spring pads on the rear and we are done.

I removed the vacuum lines from the t-case and sealed the inputs. I also removed the vacuum line that fed the system in the engine compartment and sealed the source. As far as I can tell the entire vacuum system for the 4x4 has been removed.

I did not drive this thing in 4x4 prior to the swap.

the mj arrived in the shop as a daily driver, no cap on the brake master cylinder. yeah yeah I know. The rear cylinder reservoir was completely empty. the rear brakes useless and non functional.

When the rig is placed in 4x4 the rear brakes lock up after about 1/4 mile?

When in 4x2 the rig is perfect.

The only thing I can think of is a vacuum is sucking down the rear brakes. E brake and all work fine. Did I miss something in the 4x4 system or is it possible that the master cylinder is bad due to the dryer reservoir?

If it is the master cylinder why doe it do it only in only in 4x4?

Silly question: are you sure it's the brakes locking up? In 4x4 on the street, you can bind up the tcase stopping the vehicle just like the brakes were locked. Usually happens if you do a few turns or in your case, I'd suspect different gear ratios accelerating the bind. The brakes work on positive pressure, all a vacuum could possibly do is retract the shoes away from the drum.

not stupid we thought of the same thing. but the axles are both factory setups out of the same rig. so they match or should and both end would be bound. the other thing is the rig still drives but drags the rear. after about 1/4 mile. the rear drums are hotter than heck. I pulled the wheel off and the lug nuts were sizzling in the snow on the driveway. the brakes were definitely locked. in 4x2 everything is smooth.

If the ratios were out the thing would start binding almost immediately, and no way could you drive it down the paved road.

whatevah did not call you stupid.

The only difference between 4x2 and 4x4, as far as the rear axle and your brakes are concerned, is NOTHING.

There is no vacuum interface with the brake system other than the power booster.

If you somehow had some components mixed between ABS and non-ABS it would still be exactly the same as in 4x4 as in 4x2.

Not calling anyone stupid, or a liar, just saying we can't see it happening because of the transfer case shift lever position.

he didn't call me stupid, either just saying that he didn't think my question was stupid.

You've really got a weird problem, though... there is no logical explanation why shifting into 4x4 locks up your brakes. A few years ago, I was mall-crawling on some snow piles at school in 4lo and parked on a pile for class. When I left, I forgot it was still in 4lo... I made it around 300 feet before it stopped. Was able to drive in reverse enough to clear the bind and shift back into 2Hi. That's why I suggested it initially, in 4hi, I could see you getting up to 1/4mi out.

Do both sides lock up, or is it just one side? If you can, try putting it on a lift (or a bunch of jackstands) to get all of the tires off the ground and put it in gear. Don't touch the brakes... and see if it locks up at all. If not, maybe it's something vacuum related with the master cylinder needing a vacuum to release the pressure from the rear lines? I've never heard of that, but it's all I can think of.

Good luck!

I meant that his question was not stupid.: he said he had a silly question, i said not because i had had the same thought. clarified that in my editing of the wording. sorrry for the confusion.

thanks, at least you weren't confused by the previous reply.

i have the rig up in the garage right now to try that very thing. i actually have a new master cylinder because we had to replace the cover for the old one, i know, i know too detail oriented. I may just throw it in

I was going to try it in the garage on jack stands, seems like the rig would do it two wheel drive also if the master cylinder was bad.

i called Brian Olson, he is the shop manager at one of my local garages and a SAE mechanic and an xj nut and all around decent guy.

He is also scratching his head.

It is both brakes, i couldn't see why the brakes would be so hot if it was the gearing. and the front doesn't drag at all gearing woudl definitely lock up the front too.

rockhounds4x4 said:

I meant that his question was not stupid.: he said he had a silly question, i said not because i had had the same thought. clarified that in my editing of the wording. sorrry for the confusion.

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It appears that I am the only one confused. :confused1

Could the parking brake cables be tangled up in the T-case linkage somehow? Thats the only thing I could think to cause it...

bdiamond said:

Could the parking brake cables be tangled up in the T-case linkage somehow? Thats the only thing I could think to cause it...

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Damn good thought!

Don,t MJ,s have some sort of load sensing device on the rear brake system ?It really shouldn,t make a difference what rear axle is in there 35 44 8.25 or 20, but something is going on.
Have you inspected the rear shoes for broken parts and adjusted them properly ?

Wayne

Did you open the diffs? Maybe you have a damaged bearing or a chipped spider gear? A small chunk of metal could be floating around and just happen to jam at the right time, but that would be more intermittent. Just a thought, you've got me hooked! If your master was empty, could it be possible for the wheel cylinders to bind or react and stock when hot? Maybe as you drive it, the cylinder heats up and seizes or something...

Wayne Sihler said:

Don,t MJ,s have some sort of load sensing device on the rear brake system ?It really shouldn,t make a difference what rear axle is in there 35 44 8.25 or 20, but something is going on.
Have you inspected the rear shoes for broken parts and adjusted them properly ?

Wayne

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There is a proportioning valve in the rear, but usually the problem with that is not getting it bleed and having poor braking in the rear.

It could be axle bearings getting hot. That will make the drums hot as well. Pull the diff cover and smell if the fluid has gotten hot.

Did you ever resolve this?

I also would like to know