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Mushy Brake Pedal / Brake Light on Issue

J

jnewell

NAXJA Forum User
Location
Eastern Mass.
Greetings.

Replaced both rear wheel cylinders as well as front hoses, calipers, pads and rotors.

Bleed the master cylinder on a bench, reinstalled. Bled the brakes, first by foot (two person method), then with pressure bleeder. "Bled" the combo valve by cracking the flare nuts to let fluid bleed out.

I've run about two quarts through the system and still have (1) a mushy pedal and (2) the brake light on (verified due to MC problem, not PB).

The only thing I can think of is either the combination valve needs a reset or needs replacement.

I've tried repeated quick stabs on the brake pedal with no effect on the light/valve status.

Other than replacing the combo valve, I'm at a loss. The vehicle has only about 35k miles on it and gets very carefully cared for so it's hard to imagine that the combo valve or the MC have gone bad, but that's basically all that's left unless you guys have some suggestions - which would be appreciated.

Many thanks.
 
Forgot to mention - this is a 2000 XJ 4.0 Sport - standard brakes, no ABS.

[Note to mods - apologies for replying to own post - didn't see an edit button.]
 
I recently beat this subject to death on the soft pedal and combo valve, and other earlier causes in this huge thread. I think all the questions and answers are here:

http://www.naxja.org/forum/showthread.php?t=1124404&highlight=soft+brake+pedal&page=3

Sounds to me like you need to recenter the combo valve, use a tool to keep it centered and re-bleed once again. I take it you bleed till no more air came out??? My earlier problem was a bad, new, rear wheel cylinder letting air in slowly but not leaking fluid, but it did keeping bleeding air after a short drive and re-bleed. I am down to just the combo valve itself (it may need new seals) now, and I just have not had the time yet to try the fixes posted there in last 2-3 pages of posts regarding centering the valve and holding it there with a tool while bleeding and replacing the seals.....
 
I wasn't sure by reading what you wrote--I see you bled the MC. Did you also check for an internal leak? Seal off the ports and try to stroke it? Piston in it shouldn't move unless there's a bad O-ring or other fault inside.

Anyway, just checking.
 
Ecomike said:
I recently beat this subject to death on the soft pedal and combo valve, and other earlier causes in this huge thread. I think all the questions and answers are here:

http://www.naxja.org/forum/showthread.php?t=1124404&highlight=soft+brake+pedal&page=3

Sounds to me like you need to recenter the combo valve, use a tool to keep it centered and re-bleed once again. I take it you bleed till no more air came out??? My earlier problem was a bad, new, rear wheel cylinder letting air in slowly but not leaking fluid, but it did keeping bleeding air after a short drive and re-bleed. I am down to just the combo valve itself (it may need new seals) now, and I just have not had the time yet to try the fixes posted there in last 2-3 pages of posts regarding centering the valve and holding it there with a tool while bleeding and replacing the seals.....
Click to expand...

Thanks, I will check that thread. Yes, bled till no air in the fluid. There is clearly air somewhere in the system.

cnugent said:
I wasn't sure by reading what you wrote--I see you bled the MC. Did you also check for an internal leak? Seal off the ports and try to stroke it? Piston in it shouldn't move unless there's a bad O-ring or other fault inside.

Anyway, just checking.
Click to expand...

Good point - no, I haven't checked the seals in the MC. I'll see if I can get some plugs locally, or order some. I know the thread size from the fittings I used to bleed the MC.
 
If you have a hard pedal with the engine off, like mine, then the MC is OK. Mine is only soft with the engine on and vacuum applied, as were others here that showed up in the other thread... that left us with the combo valve. If it is mushy with the engine off, then it might be MC itself and the test suggested would be a good one.

jnewell said:
Thanks, I will check that thread. Yes, bled till no air in the fluid. There is clearly air somewhere in the system.



Good point - no, I haven't checked the seals in the MC. I'll see if I can get some plugs locally, or order some. I know the thread size from the fittings I used to bleed the MC.
Click to expand...
 
If you have bled the breaks that much and are sure you don't have a leak, my money is on improperly assembled/adjusted rear brakes. If they aren't held in relatively close approximation, (via star adjuster) then they have to move quite a bit every time you press the brakes before they make contact with the drum. That will both throw a brake light code and give you spongy brake pedal.
 
I did readjust the rear brakes with the star adjuster but I may check that again.

Question for everyone on the MC: how easy is it to damage the seals while bench-bleeding? I am wondering if I cycled the piston too far while bleeding it and damaged the seals. I need to find some M10x1.0 and M12x1.0 plugs to test the MC.
 
At least 30% of the time the MC failed, I've found it is from casting flaws in the cylinder walls. Sometimes quality control lets one through that has a microscopic layer of metal over an air bubble in the metal walls of the cylinder that eventually fails.

Sometimes, seldom but it does happen, there is a piece of trash that blocks an exit port from the MC. The dual pistons don't move like designed and/or fronts or rears get low pressure/volume.

Just a couple of possibilities, other than the obvious, that you have already tried.

It was also mentioned (Jworth) that the rears need to be close to adjusted. This was actually my first guess also. You have a large unbalance because of excessive rear shoe play.

The brake pressure switch/brake light will come on from low overall pressure or a large imbalance in the system front and rear.
 
OK, still a bust here.

I reset the combo valve - confirmed by brake light performing correctly. The piston in the valve is locked with one of those tools with the teat to prevent movement.

I took up 100% of the adjustment in the rear wheels, actually locking them up tight with the star adjuster. (I have become pretty adept at loosening them in the process, which was not a skill I had previously developed in more than 40 years of turning wrenches on vehicles).

I've run at least two more quarts of brake fluid through the system with no visible air (using a Motive power bleeder).

I am just plain stumped...suggestions welcome.
 
I was just going to suggest what garr did. Check to make sure the bleeder screw on the caliper is mounted towards the top. The opposite side caliper WILL still install correctly, only with a low-mounted bleeder. I've seen it done.
 
A picture is worth at least 1K words...this is the right (passenger) side caliper. Or at least that's the side it's mounted on...

XJ-1_zpsvanh3hjm.jpg
 
Last edited:
I discovered the hard way 8 weeks ago, there is front and rear side mounted caliper for these jeeps, and you need to get the correct set!!!! If you install a front mounted on rear mounted hardware that is what happens.

Why some XJs have front mounted and some have rear mounted I have no idea. Might be a 2wd versus 4wd drive issue. So it is not a drivers side mounted on the passenger side issue. I got schooled on this by a new guy at AZ of all places, LOL

But that sure will trap air that never leaves in bleeding steps and makes the brakes mussy and nearly useless.
 
I believe you just need to swap the calipers side to side. A left caliper will bolt to the right knuckle but the bleeder will be upside down. Like you have now.
IE put that caliper on the other knuckle. Bleeder must be the highest point. Air is lighter than brake fluid
 
Dragonlich1961 said:
I believe you just need to swap the calipers side to side. A left caliper will bolt to the right knuckle but the bleeder will be upside down. Like you have now.
IE put that caliper on the other knuckle. Bleeder must be the highest point. Air is lighter than brake fluid
Click to expand...

That is what I thought at first, but I was later told and I think shown that the side to side swap will not install. But the front versus rear mount issue does cause the bleeder screw location issue in his photo.

In other words the parts house gave him the wrong caliper(s)
 
I had ceramic pads on my volvo and it left the pedal extremely mushy. I switched to semi metallic pads and the pedal was so much more firm. It's crazy what just pads can do.

Sent from my HTC M9 using Tapatalk
 
Ecomike said:
That is what I thought at first, but I was later told and I think shown that the side to side swap will not install. But the front versus rear mount issue does cause the bleeder screw location issue in his photo.

In other words the parts house gave him the wrong caliper(s)
Click to expand...

I'll bet you the XJ that they bolt right up. ;) There never were factory rear disk brakes for the XJ so there's no chance that these are rear calipers. If I get time, I'll try to swap them today.
 
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