Brake upgrade gone awry - diagnosis help please!

[What Rd]

I lost a weekend of wheeling to this head-scratcher, so I'd really appreciate some help in getting this sorted out and corrected.
On my 93 XJ, 4.0, AW4, non-ABS with a swapped-in D44 with a TSM disc conversion, I replaced the single diaphragm booster with a yard-sourced double-diaphragm booster from a 96 XJ. I also installed a new master for a 96 XJ. Before this upgrade, I was getting braking that was pretty weak - just barely livable. Immediately after the install, I had to push almost all the way to the floor to get any resistance on the pedal. Adjusting out the rod inside the booster that pushes into the master cylinder seemed to correct that problem. The braking at this point was much improved, although still biased too much to the front. I had planned to open the proportioning valve and remove the o-ring as has been described here many times - but after my wheeling weekend.
On the morning of the first day of event runs, and before even getting past the edge of town, my brakes started to drag. Every time I used the brakes, the harder they were sticking in the applied position. This continued until I could only move by driving the engine hard against the nearly locked up brakes.
After relieving some of the pressure on the front lines by opening the bleeders, they returned to normal on/off function - or so it seemed. After a few applications, the continual increase in pressure on the front brakes became obvious. This repeated three times. A review of possibilities involved rebleeding the master, rebleeding all the lines, opening and cleaning the proportioning valve, and checking that the master was pulling on the fluid on the upstroke. Still, with one caliper off and a clamp applied, the pressure from that caliper went across into the opposite front caliper - rather than back up into the master.
Believing this to be a problem with the proportioning valve, I removed the o-ring for the rear circuit. That had no beneficial effect. Then I swapped out the proportioning valve for one from a 2004 TJ Unlimited. Still no beneficial effect.
I'm at a bit of a loss to figure out what's wrong. Suggestions? Anyone encountered this behavior before?
Thanx in advance.

 

[Led]

you have the rod adjusted too long back it off and add a residuial(sp) valvefor the rear discs

 

[What Rd]

you have the rod adjusted too long back it off and add a residuial(sp) valvefor the rear discs

That occurred to me early on, but I guess I dismissed it as a possibility. I'll definitely try that tonight.

So who can explain to me where a residual valve would get plumbed in, and why I need one? I know my need to understand can be a pain, but it's there so we just make the best of it. I thought some systems used a residual valve to keep some amount of fluid pressure on the rear drum circuit to keep the drum brake's springs from collapsing the wheel cylinder. Now that the drums are gone, why would a disc circuit need that residual pressure? Also, wouldn't this just be the case when using the stock proportioning valve? I swapped in a proportioning valve designed for a 4-disc system. Wouldn't this already have the proper amount of pressure for the rear circuit built into it?

Thanx again!

 

[old_man]

Beware that with the dual diaphram you have to have a seal between the MC and the booster. If it is not sealed you will not have hardly any brakes. I ran into this doing a swap from a single to a dual diaphram booster and an E350 MC.

 

[What Rd]

I wasn't aware of the need for a seal, but that's obviously not the problem I'm having. It's a case of my front brakes building up pressure with each push of the pedal that does not release - it only builds up until the brakes are locked.
How did that E350 master work out for you, and what calipers are you running with it?

 

[rws]

Had a similar problem. When you bleed the brakes the compensating port in your master may be plugged. The solution is to replace the master cyclinder.

 

[Dingo509]

any possibility that you kinkied a line while doing the install? my logic for this; a brake system will but out over 1000 psi while the brakes are being applied but the only thing to push the pads away from the rotor is the runout of the rotor itself. if the line was kinked or smashed the 1000 psi could be enough to push fluid past the kink but the small pressure when releasing the brakes would not be enough for the brakes to release, kinda working like a check valve

as far as the residual valve you shouldn't need it, they where more designed for stree rods where the M/C was under the floor, below the caliper whithe the hieght difference in the XJ's (especially lifted) you should not need the residual valve, gravity will do it for you.

Hey Old Man, i too am curious about your swap to the Dual booster and e350 master cylinder. how did you get them to seal?

My brakes work fine now (stock 91 booster, 1 1/4 M/C, 1/2 chev front calipers, s-10 rear both on 11 or 12" rotors) i can lock my 33 swampers on trailready beadlocks at just about any speed but the pedal is kinda stiff.

Dingo

 

[What Rd]

Had a similar problem. When you bleed the brakes the compensating port in your master may be plugged. The solution is to replace the master cyclinder.

So where is this compensating port and is it a likely candidate where, as in my case, the master cylinder is new rather than a rebuild?

 

[What Rd]

any possibility that you kinkied a line while doing the install?

as far as the residual valve you shouldn't need it, they where more designed for stree rods where the M/C was under the floor, below the caliper whithe the hieght difference in the XJ's (especially lifted) you should not need the residual valve, gravity will do it for you.

No - the lines are in good shape. I didn't mess with the lines below the proportioning valve at all, and the lines from the proportioning valve to the master are new and smoothly radiused.

I still dont get why the pressure to my back brakes is so much less than the fronts and what a residual valve would do to affect that. Any other help?

 

[old_man]

I cut an aluminum piece and an neoprene gasket and sandwiched them between the MC and the booster. It's not real pretty, but it worked.

 

[Dingo509]

No - the lines are in good shape. I didn't mess with the lines below the proportioning valve at all, and the lines from the proportioning valve to the master are new and smoothly radiused.

Well then unless you got anything trapped in the lines that should not be the problem. What do the pads look like? front and rear?

I still dont get why the pressure to my back brakes is so much less than the fronts and what a residual valve would do to affect that. Any other help?

That's what i mean a residual valve will not help. What calipers are you using in the rear? I had the exact same problem when i was running the Caddy calipers because the e-brake adjuster was froze up inside the piston so after the pads wore down a little the piston had to move far enough and that took all the fluid being moved so it wouldn't build pressure. As soon as i went to S-10 front calipers (in the rear) the problem went away.

Make this easier, list all parts of your brake system that have been swapped

Dingo

 

[rws]

The compensating port is in the master cyclinder and it allows the brake fluid to move back and forth in the system as it heats up. If plugged, the fluid in the system will heat up (expand) and apply the brakes even though you are not touching the pedal. Do a google search and you should get a few hits on brake cyclinder diagrams that explain in more detail. Good luck and post your results.

 

[What Rd]

The compensating port is in the master cyclinder and it allows the brake fluid to move back and forth in the system as it heats up. If plugged, the fluid in the system will heat up (expand) and apply the brakes even though you are not touching the pedal. Do a google search and you should get a few hits on brake cyclinder diagrams that explain in more detail. Good luck and post your results.

I had put about twenty miles on these brakes before they acted up like this. They'd been fine, if unspectacular, until they got all squirrely. But it was really hot this past Friday, and as the brakes began to drag, they generated a lot more heat before I got it pulled off the road and turned around. Back in the shop and out of the heat, it took many more pumps before it would begin to drag and then lock up again. So I'm thinking this compensating port may be my culprit.
Didn't get a chance to work on it tonight, but am anxious to get to the bottom of this.

 

[What Rd]

What calipers are you using in the rear? I had the exact same problem when i was running the Caddy calipers because the e-brake adjuster was froze up inside the piston so after the pads wore down a little the piston had to move far enough and that took all the fluid being moved so it wouldn't build pressure. As soon as i went to S-10 front calipers (in the rear) the problem went away.

I have the Caddy calipers in the rear. The e-brake seems to work fine. Does that say anything about whether the e-brake adjuster might have this problem?

 

[Dingo509]

I have the Caddy calipers in the rear. The e-brake seems to work fine. Does that say anything about whether the e-brake adjuster might have this problem?

No just because the e-brake works doesn't mean its adjusting like it should. you said earlier that you have like 20 miles or so on these brakes so i'm hinking your having problems with the e-brake adjusters. try this;
first remove the cable from the lever on the back of the caliper, then remove the spring. now BY HAND move the lever as far as you can like you are applying the e-brake, and then the leave the lever in this postion, what you just did was move the pads closer to the rotor and took out any space. this should cause a temporary increase in rear pressure as now the piston only has to apply pressure instead of move and then apply pressure try this and report back then we'll address the front.

Dingo

 

[What Rd]

No just because the e-brake works doesn't mean its adjusting like it should. you said earlier that you have like 20 miles or so on these brakes so i'm hinking your having problems with the e-brake adjusters. try this;
first remove the cable from the lever on the back of the caliper, then remove the spring. now BY HAND move the lever as far as you can like you are applying the e-brake, and then the leave the lever in this postion, what you just did was move the pads closer to the rotor and took out any space. this should cause a temporary increase in rear pressure as now the piston only has to apply pressure instead of move and then apply pressure try this and report back then we'll address the front.

Dingo

I'll definitely try this, but it'll be a couple of days at least before I get a chance. I still haven't been able to get my Jeep back in the garage due to the "project" my wife brought home for me to do for her.

 

[corbinafly]

you have the rod adjusted too long back it off and add a residuial(sp) valvefor the rear discs

I had the same symptoms when I changed my MC and booster. I had the secondary push rod adjusted out about 3/16" too far. Brakes were fine for a few miles then gradually started to drag and kept getting worse. Luckily I was close to home so I turned around and fixed it.
Your pedal should travel about 1/8" before you feel any resistance. That way when you're off the pedal, the push rod isn't pushing on the MC piston.

K

 

[What Rd]

I had the same symptoms when I changed my MC and booster. I had the secondary push rod adjusted out about 3/16" too far. Brakes were fine for a few miles then gradually started to drag and kept getting worse. Luckily I was close to home so I turned around and fixed it.
Your pedal should travel about 1/8" before you feel any resistance. That way when you're off the pedal, the push rod isn't pushing on the MC piston.

K

This will be the first thing I try - if I EVER get my Jeep back in the $#*&@!! garage.
It'll be so nice to have brakes again . . . someday.
:tear:

 

[MudDawg]

The last thing you need is a residual valve...they are designed to hold a small amount of pressure to the rear DRUM brakes of single circuit brake systems. And what actually retracts the piston in a caliper is the flex of the seal after pressure is released. It is a very small amount, but enough. If your mount hardware and attachment points are in good shape, it's all you need. If you find the inner pads wearing too fast...fix the hardware....or remove the dust shields that are packing up with mud.

 

[bj-666]

i would have to say that it is the rod adjusted too far out. it takes a little while to get it in the right position but work with the rod and you should be fine