Have to pump the pedal after a rear disc brake install?

Okay, I have had it with this problem. I installed a rear Dana 44 back a few months back that had the disc brake conversion already done from an El Dorado and installed longer brake lines while I was at it. The brake system was bled and everything worked good for about one day. Well the problem is that everytime I go to stop, the pedal goes down way far and the brake light comes on, and in order to get any stopping power, I have to let my foot off the brake and re-apply them, and then it stops great! Well I am so tired of having to pump my brakes and having to teach people how to stop if they ever need to drive my XJ anywhere.

This whole time I thought that there was some air stuck secretly somewhere within my system and that it needed to be bled again. So I went to a reputable brake shop to have a complete brake flush to ensure that there was no air whatsoever in my system, but I am still having the same issues. The man at the shop asked me who did my brake job and if they had modified the proporrtioning valve, which I told him had not been touched.

I have since read a few write-ups on proportioning valve modifications and wonder if anyone here has had improved braking after removing the O ring on the proportioning valve. I hope that I have been clear on the problem that I am having, and have now ruled out air in my system. The guy at the shop said that the rear calipers have pistons that are much bigger than the stock drum brake pistons and that may be why I am having to pump the pedal to be able to stop.

Will removing the O ring improve the distribution of fluid to the rear or will it not do anything to fix the problem that I am having? Please give me your suggestions or share your experiences with me. There are a lot of threads on the subject of ZJ proportioning valves and other similar ones, but not one that directly deals with just people that have removed the O ring. The only write-up I have found was not on this forum, but here is the link. I just want to see if anyone here has personal experience with doing this. Thanks.

http://www.lunghd.com/Tech_Articles/Suspension_Steering_Axles_and_Brakes/Proportioning_Valve_Mod.htm

I don't think the valve is going to be your problem. The pistons on my 8.8 are small and everything works great. My buggy has 2 piston callipers in the front and big 1 piston chevy calipers in the back. My buggy does the same thing that your are talking about. The problem is the master cylinder is too small. I have read that some use a vette or E-350 master cylinder, But have not tried any myself.

It sure sounds like air in the system. I looked at your profile and you have a 99. I put a 99 XJ master cylinder and vacuem booster on my 88. My brakes work fine, but I'm only running 31's. You may have air in the fronts. It will affect it also.

Cox89XJ said:

It sure sounds like air in the system. I looked at your profile and you have a 99. I put a 99 XJ master cylinder and vacuem booster on my 88. My brakes work fine, but I'm only running 31's. You may have air in the fronts. It will affect it also.

Click to expand...

Yeah, I agree that it sounds like it is air, but after the brake flush, the shop guy assured me that there was absolutely no air in the system, so that has to be ruled out, right?

Adjustable Proportioning Valve.

i've heard mixed result's on modifying the stock proportioning valve. might to be much or not enough again.

there's a HUGE write-up on the front page of Pirate about basic brake tech. about 3 day's worth of reading.

Check the rotors for run-out,check the axles for end-play.Consider some residual pressure valves!

Yep, it needs a residual valve. Without that valve the pistons will retract too far and you have to burn up pedal travel pushing them back out again.

When I did the conversion on the Heep, I used Monte Carlo calipers. The extra volume of the calipers makes the pedal travel farther...I don't have to pump the pedal to have brakes, but it sure travels farther..you may still have air in the system...the best way to get it out is to gravity drian them...take the cap off the MC....crack the bleeder port on the passenger side...and watch the fluid drain...keep the MC full of fluid...give it at least 5 minutes... close the bleeder and repeat on the driver side...if you have air in the system, tiny bubbles will come out with the fluid...it don't take much air to make a spongy pedal.

The other possibility is a bad MC...if you pump up the pedal and hold it down without a lot of pressure and it holds, you are ok...if it slowly goes to the floor, the MC is bad....

Another thing to check is the bleeder screws. make sure they are on top. Sometimes people put the calipers on the wrong side and the bleeder screws end up on teh bottom, which traps air in the top of the caliper. You can't get that air out with normal bleeding.

muddeprived said:

Another thing to check is the bleeder screws. make sure they are on top. Sometimes people put the calipers on the wrong side and the bleeder screws end up on teh bottom, which traps air in the top of the caliper. You can't get that air out with normal bleeding.

Click to expand...

X2 thats what I would check...with my 93 and disc brakes all around, my pedal is really stiff, just like it was with drum brakes. I had no need to upgrade the booster on mine, but I did swap out the proportioning valve...which only made it stop better, it didn't have anything to do with pedal pressure.

It sounds like air is still in the system. I did the disc brake conversion last week and noted that the only difference between the 96 xj prop valve and the zj prop valve was that the zj's spring was about a half inch shorter so I just swapped it out. Some people have cut their spring down instead of swapping it and some people have removed the o-ring. I don't know what year your xj is but the 95+ xjs already have the dual diaphragm booster. Some people with earlier xjs have upgraded that as well.

Not to be confused with the Heep....my '91 MJ got a 8.8 exploder axle with disc brakes....no problems at all, full pedal, killer brakes...the pistons in the exploder calipers are small......unlike the big honkin pistons in the caddy calipers.

It sure sounds like I am the lone wolf here that has the El Dorado rear disc brakes. Frankly I am just tired of dealing with this issue and just want to do whatever I can to have functional rear discs. Should I just swap over to crown vic brakes or something? It seems like El Dorado calipers are just too big cause most people don't seem to have to deal with the proportioning valve to get theirs to work properly.

What year of Caddy brakes do you have? The older brakes had a much larger piston than the latter, "metric" calipers. Also, assuming you have the Caddy brakes with an e-brake, you need to do some on-line research for proper adjustment.
Most disc brakes are self-adjusting; the Caddy e-brake models are not. There is a specific procedure for setting up the e-brake, which dramatically affects piston position. After they are set up, you need to apply the e-brake frequently (manual says every time you park) to insure the pistons are properly set in the bores. If you ignore this step, eventually your pedal will get lower and lower, the e-brake internals will freeze up, then a complete caliper rebuild becomes necessary.
In order to bleed mine, I have to remove them from the axle, hold them up high, and insure the bleed valve is at the absolute top of the cylinder, then bleed away---I use a vacuum bleeder, but the "pump and hold" method will also work, as long as you put something between the pads.
Mine work fine, but they do require more attention than other styles of rear e-brake calipers.
For me, installing rear discs that need a specific rotor/drum combination was not acceptable, such as the Crown Vics.
The proportioning valve has nothing to do with pedal height, or firmness, unless it has air trapped in it. It does need to be replaced with a valve intended for 4-wheel discs, or "gutted", so as not to reduce pressure to the rear pads.
The MC needs to have sufficient volume to service the rear disc calipers. I find a piston size of 1.125 minimum to get the required volume, and it needs to be a 4-wheel brake application MC, which means the front and rear reservoirs are similar in size; disc brake pistons keep adjusting outwards, requiring more and more fluid to operate. Drum brake cylinders always retract the same after each application.

muddeprived said:

Another thing to check is the bleeder screws. make sure they are on top. Sometimes people put the calipers on the wrong side and the bleeder screws end up on teh bottom, which traps air in the top of the caliper. You can't get that air out with normal bleeding.

Click to expand...

I didn't want to admit it, but I am guilty of this one time. (embarrassing)

Cox89XJ said:

I didn't want to admit it, but I am guilty of this one time. (embarrassing)

Click to expand...

So am i :wave1:

It happens. The brake experts at Midas did it too.

a buddy of mine had the exact same problem and it wasnt air in the lines he swapped master cylinders i dont know off the top of my head what he went to though, i am runnin older 2.5 inch caddy calipers on the back and ford twin piston up front lookin for info on what master cylinder to swap in.

My .02 worth. The Caddy brake setup is common in the hot rod/classic car world and let me tell you, they suck A$$. My buddy just had a set on his '67 Chevelle. He put 50 miles on them and found they sucked so bad, he ditched the front and rear setup and but Baer 13" brakes all around. Hugh improvement.

My .02 worth.