Help with brake problem, please...

Help, I have no brakes! I just swapped in a ’99 Explorer 8.8 rear end (4.88’s and a Detroit…yeah!) for the obvious strength advantage and also for the disk brakes. As my tire size has grown incrementally larger the effectiveness of my stock brakes has diminished. With 34” LTB’s the brakes were pretty marginal. When everything was installed and we bled the brakes, we could not get a firm pedal. Ok, maybe the master cylinder’s bad, everything else that came with the Jeep has gone bad and it probably has too. So we install a brand new MC and the pedal is mushier than before. The best we can do is get a somewhat firm pedal after 8-9 pumps and if you hold it, you can feel the pedal go down! This is with the engine off and no vacuum to the booster.

At this point we have run over ½ gallon of brake fluid through the system. The fluid runs clear without any bubbles at all four corners. There are no leaks at the calipers or line connections. We adjusted the piston length from the booster to MC per instructions obtained from a replacement booster at Auto Zone. The piston will pull out of the booster, this doesn’t seem right…is this a problem? If it’s a problem is it relevant to pedal firmness? I think it’s still a problem with the MC, and I have been considering an upgrade anyway but I would like to have a clear understanding of the problem before introducing other parts in the mix.

Any insight would be appreciated!

TIA
Patrick

Did you bench bleed the new MC before installing it? They MAY be why the pedal was mushier than before.

I have no other great wisdom as to why your pedal is mushy in the first place, im not a brake guy. You also might want to do a search on proportioning your brake pressure now that you have disc brakes. Remember, the stock prop valve isn't designed for 4 wheel disk.

-jm

HTeK said:

Did you bench bleed the new MC before installing it? They MAY be why the pedal was mushier than before.

I have no other great wisdom as to why your pedal is mushy in the first place, im not a brake guy. You also might want to do a search on proportioning your brake pressure now that you have disc brakes. Remember, the stock prop valve isn't designed for 4 wheel disk.

-jm

Click to expand...

Yes, I bled the MC

You will also need to adjust/get a new proportioning valve as well due to the old one will not work properly as the old drums used less fluid then the disks will.Yes there is a problem if the piston comes out of the MC.as it will cause the pressure to the brakes to bleed off through the piston insted of goingg to the brakes.Hope this helps and good luck.

SEAN.

I was planning on doing the proportioning valve mod that allows better fluid distribution. But I don't think this will affect how firm the pedal is, will it? I didn't want to make too many changes all at once, I find that sometimes it makes troubleshooting easier, and sometimes not...

The prop. valve shouldn't give the mushy feeling you are getting but the piston comming out of the MC definatly will I would recomend the MC from the same Vehicle that you used for the donor axle(or as close as you can get,as the piston lengths might be different)and at the same time grab the prop. valve.

SEAN.

I'm thinking about doing this

http://www.4x4wire.com/jeep/tech/brakes/xj_wjbrakes03/booster/

Opinions?

BTW, this is an '89, 4.0, AW4, no ABS, D30, 8.8 (Ford)

That seems like the easiest way to go, I would do it.

SEAN.

Well, I went ahead and did this:

Iminocca said:

I'm thinking about doing this

http://www.4x4wire.com/jeep/tech/brakes/xj_wjbrakes03/booster/

Opinions?

BTW, this is an '89, 4.0, AW4, no ABS, D30, 8.8 (Ford)

Click to expand...

and it still didn't help! After much more deliberation, anxiety, and even more brake fluid, we (my brother-in-law and I) decided to seek professional advice. After being turned away by 4 shops, a local shop that specializes in custom steering and brakes puts it on the lift, takes one look at it and says "your calipers are reversed".

Seems when you reverse them little air pockets form that you can't bleed out. How this happened was that I took the rear end apart when he wasn't there, and he put it together without me checking it.

The only reason I'm sharing this embarrasing info is so one of you won't go there too....

Cheers

Oh, the brakes work better than ever now :laugh3: