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Disc Brake Woes..

X

Xhavier_5478

NAXJA Forum User
Location
Colorado Springs, CO
I am in need of some advice, my cherokee has the Crown Victoria rear disc swap on a Dana 44, and the WJ big brake swap up front. I set it all up, added ZJ spring and internals to my prop valve, bled the system, and things were great. Since this is a jeep I've been working on about 5 different projects on it at anyone time, well it sat for a few weeks while I worked on other issues, when I went to drive it the next time the brakes were dragging, so I pulled the rear wheels and both calipers had constant pressure, so I pressed them back in with a c-clamp and did the same in the front, drove fine after that. Let it sit for a week or two again this time only the fronts, so I figured they were bad calipers from the Pull and Pay so I should replace them and get new ones, did that, bled the system and they were fine, fast forward a few months my jeep sat while I was doing my drive train swap, it did it again I forced the rear piston in and it was fine.

Last night after the jeep has sat 3 weeks, I actually had to bleed off some fluid from the rear caliper to force the piston back in, and the front wouldn't retract till I let some fluid out of it also, after that all are fine.

So I have found that driving as little as 3 miles will cause the pressure to build up in the rear calipers also, and make the engine strain to overcome the braking force.

I had to remove the ZJ spring and piston from the prop valve and swap in a xj one, the brake light came on but it was able to drive home with out dragging the rear wheels.

I just finished bleeding the brakes again and they were awesome for about 5 miles, and now it is dragging the rear brakes again.

Kind of at a lose for what to try next.

Does anyone run the Crown Vic disc/WJ disc combo, and did you have to go to a different Proportioning valve all together?
 
Sure sounds like the master cylinder isn't fully retracting, which blocks the return port.
As you drive, the brake fluid heats and expands. With the return port blocked, the pressure builds, the brakes are slowly applied and will eventually come to a stop.
The classic test is when this happens, crack a brake line, at the master cylinder, just long enough to release the pressure, tighten the line and see if the brakes now are free.
It doesn't cost much to give it a try.
 
lazyxj said:
Sure sounds like the master cylinder isn't fully retracting, which blocks the return port.
As you drive, the brake fluid heats and expands. With the return port blocked, the pressure builds, the brakes are slowly applied and will eventually come to a stop.
The classic test is when this happens, crack a brake line, at the master cylinder, just long enough to release the pressure, tighten the line and see if the brakes now are free.
It doesn't cost much to give it a try.
Click to expand...

I am with this 100%.

Check and re-check all push rod lengths from the break pedal to the master cylinder. Also, if you had the brake booster off, check that the 1/2" spacer is in place between the booster and the fire wall. I do not know if your year XJ require this spacer but my 1996 XJ do require one as well as several other years model.
 
I fixed this on someones car just like 4 days ago.

Pop the master cylinder off. There is a rubber o-ring on the end of it where it inserts to the booster, remove that.

Look inside the booster, and you'll see a rod with a threaded tip.

IMG_20150530_171115252_zpslisdzude.jpg


Insert the master without the rubber gasket, and you'll feel it hit that rod. It shouldnt. Thread it down like 1/2 to 3/4 turn at a time until it just barely doesnt hit anymore, and then shorten it up 1/2 turn more, re-install the rubber O-ring and bolt it all back together.

I ended up removing it and grinding the tip a little bit to make it work.

IMG_20150530_171107519_zpsqbacezmk.jpg



If it works out for you, send a message to Blaine at Black Magic Brakes in thanks, he walked me through the procedure on the phone.
 
Often our problems have very simple solutions. You just have to know to go looking for them.
 
Xhavier_5478 said:
Cal you are the man! It was way out of adjustment and my brake light switch was forcing the pedal in even more..
Click to expand...
Was the master cylinder changed recently? Or did it develop this problem after being in service for a while?
My point is the pushrod adjustment is ordinarily done once, when the MC is mated to the booster and should not need to be done again.
If your MC needed adjustment, after being in service, I wonder why?

Several years ago, when setting up the ZJ rear discs and the front Black Magic brakes, I tried several MCs, looking for the best pedal.
https://www.flickr.com/photos/lwade/sets/72157616348032595/
Everyone of the MCs needed the pushrod adjusted, before using.
It takes a bit of time to do correctly.
Funny, it turned out the stock MC was the best in both pedal height and needed pedal pressure.
 
Where's the "like" button on this thing?

So awesome when someone shows up with a weird problem and there's someone out there who's just like, "oh, yeah, just do blah, blah, and blah and you're all set." AND THEN IT WORKS.

It's almost like that's the whole reason forums like this one exist . . . or something.
 
lazyxj said:
Was the master cylinder changed recently? Or did it develop this problem after being in service for a while?
My point is the pushrod adjustment is ordinarily done once, when the MC is mated to the booster and should not need to be done again.
If your MC needed adjustment, after being in service, I wonder why?

Several years ago, when setting up the ZJ rear discs and the front Black Magic brakes, I tried several MCs, looking for the best pedal.
https://www.flickr.com/photos/lwade/sets/72157616348032595/
Everyone of the MCs needed the pushrod adjusted, before using.
It takes a bit of time to do correctly.
Funny, it turned out the stock MC was the best in both pedal height and needed pedal pressure.
Click to expand...

The MC was mated to another vacuum booster and never adjusted at install time, so pretty much ignorance on my part.

Also the brake light switch had been forced all the way in, which shortened the pedal travel some also.
 
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