WJ booster/MC in an MJ - spongy pedal feel.

[Renegade XJ]

We have a brand new booster and MC from a WJ installed in an 88 MJ. Stock prop. Valve, stock brakes all the way around (rear D44 w/drums) and used the 1/4" aluminum spacer from the WJ on the MJ. We bench bleed the MC with no issues, we have bleed the bleed the brakes at least 6 times, completely flushed out the system at this point, but we still have a very spongy pedal feel and it will not lock up the tires at all (33" Duratrac's).

We are totally confused here. The MC has been bench bleed twice BTW. We defenitly don't have any leaks. I just don't see why it's not working right. I have the same set-up in my XJ and it works great. Only differance I see between an MJ and XJ is that the MJ has a load transfer valve in-line for the rear brakes, could it br possible that there is sir trapped in there? I dont see a bleeder for it.Any advice?

[cheapcherokee93]

I would eliminate the load sensing prop valve and see if that helps any, I did that when I swapped in a 98 XJ booster into my 88 MJ and the braking power is amazing! Granted I also have rear discs but yours shouldn't be much different with drums.

[N8N_99xj]

did you do a brake job at the same time? I was going nuts when I changed the front brakes on my XJ because the damn thing wouldn't stop, apparently it'd been stopping on the front brakes only for ages and just having new, non-worn-in pads and drums exacerbated the problem. The shop I took it to (ran out of time, needed it back on the road) said that the drums were way out of adjustment. Apparently you have to get the adjustment close for the self adjusters to work. Probably explains why my fronts wore out so fast too.

[xjtrailrider]

Originally Posted by cheapcherokee93

I would eliminate the load sensing prop valve and see if that helps any, I did that when I swapped in a 98 XJ booster into my 88 MJ and the braking power is amazing! Granted I also have rear discs but yours shouldn't be much different with drums.

This^ When I installed the 96 XJ booster/MC I also used the XJ prop valve and eliminated the load sensing valve and the second line going to the back of the truck for that load sensing valve. That extra line will get air in it and its nearly impossible to get it all out giving you a spongy pedal.

Its simple to eliminate. There is a "T" on the main line, one half goes to the rear axle, the other to the load valve, you can put a coupling in place of the "T", its just above the load valve. Then just follow the second line back to the front and remove it as you go. Then just swap the MJ prop valve for the XJ prop valve or if you have disc use a 97/98 ZJ prop valve and call it done. Your brakes will bleed as they should.

Eliminate this tee


With a coupling


When you get rid of all of the second line, load valve, and MJ prop valve, you will have a pile of junk like this;


The MJ prop valve has 4 lines coming out of it and 2 going in. You could plug that port where the line went into the prop valve from the load valve but I chose to replace the prop valve since I had one from a parted out XJ.

Here is a pic of the MJ valve

[Renegade XJ]

Wow, good info guys. I was suspecting it had something to do with that load sensing valve. I going to give it a shot and I'll post up what happens.

Thanks!

[Renegade XJ]

Ok UPDATE:

I got rid of the T fitting, swapped in a prop valve from an XJ. Bleed the whole system 3 times, and I still have a spongy pedal and it won't even get close to locking up the tires... Any advice?

[xjtrailrider]

Originally Posted by Renegade XJ

Ok UPDATE:

I got rid of the T fitting, swapped in a prop valve from an XJ. Bleed the whole system 3 times, and I still have a spongy pedal and it won't even get close to locking up the tires... Any advice?

I'm suspecting a bad MC. Did you bench bleed the MC before installing it?

[Renegade XJ]

Originally Posted by xjtrailrider

I'm suspecting a bad MC. Did you bench bleed the MC before installing it?

Yea, its been bench bleed...and its not leaking.....?

[xjtrailrider]

Originally Posted by Renegade XJ

Yea, its been bench bleed...and its not leaking.....?

Externally, but it could be internally, not to the booster but just not sealing well enough to allow the piston to make good pressure. In other words there could be wear to the piston cups or the bore and fluid is bypassing internally and not giving you enough pressure.

Here is a link i found that describes how a MC works, it might help explain.

http://www.tegger.com/hondafaq/maste.../howworks.html

[Renegade XJ]

Originally Posted by xjtrailrider

Externally, but it could be internally, not to the booster but just not sealing well enough to allow the piston to make good pressure. In other words there could be wear to the piston cups or the bore and fluid is bypassing internally and not giving you enough pressure.

Here is a link i found that describes how a MC works, it might help explain.

http://www.tegger.com/hondafaq/maste.../howworks.html

Cool link, I had no idea that a master cylinder could leak "internally". This might be our issue. Im going to check it out on Thursday and post up the results.

[Renegade XJ]

I haven't put in a new MC yet, but I have reading write ups and I'm a bit confused. Are the front/rear lines coming out of the WJ booster reversed compared to the MJ/XJ booster?

[Renegade XJ]

bump, any idea which port on the WJ M/C is for the front or for the rear?

[xjtrailrider]

I would answer this but it would be a guess, I used a 96 XJ booster

[Lowrange2]

They are the same orientation as any other MC. You can't really mix them up. As long as you pay attention and put the correct fitting on the correct line.

If you're still having problems I'll get a picture for you.

[Renegade XJ]

OK....Im bringing this back from the dead.

UPDATE:
We have so far installed:

NEW WJ Booster
NEW WJ M/C
used XJ prop. valve
NEW front calipers
NEW brake pads
NEW stainless steel brake lines (all 3)

We have also done:

removed the mentioned above "T" in the MJ rear brake line/hardline
bench bleed M/C
bleed the brakes about 100,000 times or more
took it to a shop (who I trust and watched them do it) and had it pressure bleed.
adjusted the hell out of the rear drums

and we still have these issues:

Pedal feel is spongy.
Brakes are not reponsive when compared to my XJ which has the same set up.
WILL NOT EVEN GET CLOSE to locking up rear tires (33's)
Had will lock up the fronts, only on wet pavement.

I dont get it. Everything is brand new new and its identical to my XJ but the damn thing wont stop.