Soft brake peddle cause

[davesdetail]

Nope, you want the MC Level (Rear end higher), to get any trapped air out of the normally elevated (With no way to bleed air out) front of the MC.




Yes, if it's brake fluid, it leaked there from under the connector, If fluid got out Air can be getting in!

Thank you, this may be a part of my Demon! Now I feel like I may be getting somewhere.

 

[bchulett]

Well, I just finished a reverse injection bleed, and I'm back to square one. I did notice on the proportioning valve some fluid under the connector on top, could this be a issue?

I changed the proportion valve... it wasn't the problem. I have a good used one in my tool bin. If its leaking around the connectors that's another issue.

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[bchulett]

Double post

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[davesdetail]

IIRC I Have heard there is an internal bypass valve in the MCs to avoid damage to the piston cups if they are over pressured. Not fully sure of,nor completely informed on this. The VacBooster increases the force on the MC pistons (multiplies the applied foot force) many times over the non boosted (engine off) pressure, and at some point the pressure could be too high for the piston seals to seal resulting in bypass, and fluid loss back to the reservoir. If the MC bore is too large or the piston seals too small this could be happening to us, just like a worn out MC, where the bore and cup seals are too worn to hold enough pressure. If this is the case, where do we find good Jeep Renix era MCs now???? I have tried what I was told was the best brand, Raybestos from Rock Auto after giving up on 3 Autozone brand new MCs.

And how do we prove it is the cause first? I guess if I knew the peak design pressure for the brakes I could install an inline HP-hydraulic gauge, about 10,000 Psig on the lines somewhere. I know it feels like my MC is not holding peak pressure, but just enough to have near-decent brakes. If I press the peddle fast enough at 60 mph I can toss an elephant through the window, but not when applied slowly to the bottom of the stroke, making me think all four new MCs are leaking internally.

Okay, I'm begining to understand that a good Renix era MC is not only scarce, but more trouble than they're worth, and the best fix is to upgrade to a dual diaphragm '97 to '01 booster/mc, or a WJ set up?

 

[dan1977p]

Okay, I'm begining to understand that a good Renix era MC is not only scarce, but more trouble than they're worth, and the best fix is to upgrade to a dual diaphragm '97 to '01 booster/mc, or a WJ set up?

That would be a good plan regardless since you are already having issues and will be throwing parts at it anyway. Was the single best thing I did for my braking.

 

[Ecomike]

Okay, I'm begining to understand that a good Renix era MC is not only scarce, but more trouble than they're worth, and the best fix is to upgrade to a dual diaphragm '97 to '01 booster/mc, or a WJ set up?

And yet I am having no problems with the Autozone cast iron MCs I installed the last 4 years on the 85 and 89 XJs and the new 4x4 white 87 XJ I bought 3 years ago that had a bad MC when I bought it.

Just the 87 Wagoneer XJ is giving me this headache. The brakes on those 2 work others perfectly for daily driver use. For heavy duty trailer towing and off road use, you maybe right. And I have had no fluid loss on the 87 mushy peddle one for years until last month when I replaced the rear wheel cylinders, MC, pads, etc. It held fluid in the MC for 3 years with no problem then lost some one day when the brake light tripped on one day and then the brake light went back out before I replaced anything put a little fluid, but never lost any since then. Never found any of the leaked fluid. That was when I started replacing the MC and rotors and WCs and pads all the way around and looking for leaks etc. But the mushy peddle had been with me for a good 3 years by then.

Still baffled that one day the rubber hose clamp test on the passenger side gave me a firm peddle finally, and yet after installing a new hose and caliper and rotor, now even the clamp does not give me a firm peddle on or off the passenger hose side. Maybe air in another part of the lines since I opened it to replace the front Passenger side caliper. But we bleed forever getting air out the last time and never got any more air out after a while, and still much.

 

[Ecomike]

Okay, I'm begining to understand that a good Renix era MC is not only scarce, but more trouble than they're worth, and the best fix is to upgrade to a dual diaphragm '97 to '01 booster/mc, or a WJ set up?

And yet none of us have solved this mushy peddle problem with a new MC. Some claim to have solved it with better air bleeding methods. But I have 3 other good XJs 85-89 with no brake problems that I rebuilt using the same supplier and parts, Autozone over the last 5 years. Granted the 85 MC/and booster is slightly diferent IIRC, but I think the other 87 and 89 are the same as the wagoneer 87 I am fighting with

 

[Matthew Currie]

I have lost track of what's been addressed here, but have you double checked front wheel bearings?

Ordinarily bad wheel bearings can cause soft pedal when running, because the wobbling rotor pushes the shoes back. I've had this where brakes go soft after going around a curve. But it occurs to me that with a single piston caliper such as the Jeeps have, perhaps that piston is pushing the rotor out even standing still. It seems a reach, but there don't seem to be many options left.

 

[Ecomike]

I have lost track of what's been addressed here, but have you double checked front wheel bearings?

Ordinarily bad wheel bearings can cause soft pedal when running, because the wobbling rotor pushes the shoes back. I've had this where brakes go soft after going around a curve. But it occurs to me that with a single piston caliper such as the Jeeps have, perhaps that piston is pushing the rotor out even standing still. It seems a reach, but there don't seem to be many options left.

I think we checked the tire mounted and unmounted side to side and top to bottom movement for that, but may need to recheck it. Thanks. I do know I wrote it off I think because the peddle is hard with the engine off and soft with the engine on while in PARK. And the peddle is harder at freeway speeds than at park or at 10 mph on the first push.

Also we got a hard peddle on two occasions clamping one of the front haoses, but it was not repeatale, It is why I replaced both good (by inspection) rotors and one caliper (that was working by other tests) when the first hard peddle showed up with the first hose clamp test on the passenger side flex hose. I think that ruled out the bearings also, at least on the drivers side?

I am really temped to try a mild vacuum on the MC using a modified old MC lid and lab Vac-pump I have. But I am worried it might suck up in or damaged the seals on the calipers or wheel cylinders pulling a vacuum on the sealed system? Old man's Vacuum suggestion has fluid filling the MC while a vacuum at the 4 brake bleeders, so the peak vacuum is very limited. I do wonder if there is a trapped air bubble in the Combo valve that defies bleeding (I have bleed it many times), as it is the only OEM part left other than some rust free steel lines. I have never replaced the fron wheel bearings. Owned the jeep from about 214,000 miles to now, now at about 286,000.

 

[Ecomike]

I have lost track of what's been addressed here, but have you double checked front wheel bearings?

Ordinarily bad wheel bearings can cause soft pedal when running, because the wobbling rotor pushes the shoes back. I've had this where brakes go soft after going around a curve. But it occurs to me that with a single piston caliper such as the Jeeps have, perhaps that piston is pushing the rotor out even standing still. It seems a reach, but there don't seem to be many options left.

Wouldn't the peddle firm up to normal after pushing the peddle just so far? This one does not. It goes from steady state mush for about 3.5 inches, to a solid wall all in one spot where the MC piston bottoms out. Before you ask, it bottoms out about 1" off the floor because I extended the VB to MC rod length to make sure it was bottoming to make sure no air could be hiding at the bottom of the MC pump chamber after a year of fighting the problem already

 

[Matthew Currie]

Wouldn't the peddle firm up to normal after pushing the peddle just so far? This one does not. It goes from steady state mush for about 3.5 inches, to a solid wall all in one spot where the MC piston bottoms out. Before you ask, it bottoms out about 1" off the floor because I extended the VB to MC rod length to make sure it was bottoming to make sure no air could be hiding at the bottom of the MC pump chamber after a year of fighting the problem already

Yes, generally a bad bearing will give you a longer pedal travel, with momentary pull to the side with the bad bearing, followed by normal hardness and braking. There is, after all, no real malfunction of the brake. It's just that the caliper piston has been pushed back further than the seals and normal runout do. It also would not go so low on pumping or a second application. It's not likely, but I figured to throw it in as one more thing to check.

I still wonder if you just haven't gotten a good master cylinder yet. I've heard, though not experienced it myself, of multiple bad ones in a row. I've never had this much trouble getting a good pedal, including on my 87, and the couple of things that it was fussy about, such as cracked front hoses and bad rear adjusters, were addressed way back. I never found an XJ hard to bleed.

The one thing that I have experienced with a bad master cylinder is the intermittent failure, in which the pedal will go low one time and not another, with no apparent cause. One thing you might experiment with is whether the low pedal occurs more frequently when applying the brakes softly than hard. That's often a MC symptom. A defective piston cup will expand and work properly when you stab it, but fail when you apply it at normal steady rate. If it's only a little bit defective and not actually worn out, it can be a bear to track down. This happened once very very long ago with a brand new Mercedes part, and was quite difficult to pinpoint.

 

[Ecomike]

Thanks. It does seem to brake better if I slam the peddle. But still mushy. Not sure what to do if the Rock-Auto Raybestos brand cast iron MC is also bad off the shelf with the same exact problem. I could sort of buy that 3 brand new durajunk MCs from AZ were bad. I may call Rock Auto for ideas and see if they have had any problems or complaints

 

[Green Mesa XJ]

Hum. Omix-Ada has a mc part# :16719.13 says 1984-1989 XJ

I wonder if it has any better quality.

But there are dozens of XJ soft brake pedal threads all over the Internet there has to be a common cause

 

[dan1977p]

Hum. Omix-Ada has a mc part# :16719.13 says 1984-1989 XJ

I wonder if it has any better quality.

But there are dozens of XJ soft brake pedal threads all over the Internet there has to be a common cause

I think the cause is a lot of us drive newer cars too and expect the same performance out of a 25 year old light duty suv with stock parts and oversized tires.

 

[davesdetail]

I think the cause is a lot of us drive newer cars too and expect the same performance out of a 25 year old light duty suv with stock parts and oversized tires.

I'm chasing the same demon Ecomike is chasing. I have a '89. My other is a '86, which I have zero problems with the brakes. I can stop on a dime, with a very hard, stiff pedal, so I don't think age is a issue. My '89 on the other hand, has a very hard pedal until the engine runs, then it goes to the floor. It does this on a Advance Auto rebuilt mc, and on a new Napa mc. Both mc's I bench bleed before and after install, with same results on both. I'm suspecting the proportioning valve, it has fluid leaking under the switch.

 

[dan1977p]

My last statement assumed the obvious things like leaks were fixed and it was bled properly and the pedal was still soft.
I've recommended replacing the combo valve in this thread, making sure it's level and bleeding it before bleeding the rest. As this thread gets older, the suggestions seem to be meeting more resistance.

 

[garr]

I'm suspecting the proportioning valve, it has fluid leaking under the switch.

I never liked that plastic switch, if its leaking replace it. For a fast check I'd try taking out the switch & installing a plug in its hole to see if the pedal feel improves.
PS: Is there any type of sealer that can be used under that switch that brake fluid won't destroy?

 

[Ecomike]

I never liked that plastic switch, if its leaking replace it. For a fast check I'd try taking out the switch & installing a plug in its hole to see if the pedal feel improves.
PS: Is there any type of sealer that can be used under that switch that brake fluid won't destroy?

I wonder if a bad switch can not leak externally but leak internally, or trap a bubble in the switch guts or???? Mine does not leak on the outside. I have bleed my combo valve a got 3-4 dozen times while bleeding the entire system front to back and back to front.

Sealer? Maybe an anaerobic thread-locker when dry? Or an epoxy which is iffy. Might tray a fuel line thread sealer paste, industrial plumbing duty?

 

[Ecomike]

My last statement assumed the obvious things like leaks were fixed and it was bled properly and the pedal was still soft.
I've recommended replacing the combo valve in this thread, making sure it's level and bleeding it before bleeding the rest. As this thread gets older, the suggestions seem to be meeting more resistance.

I have seen no reason to replace the combo valve on mine, which would require me to make new steel lines and figure the tooling out out due to the lack of space to get at it, since mine works and does not leak. I researched the combo valve internet wide and decided it could not be the problem based on test data I have. No one has reported fixing a mushy peddle with just a new combo valve. They have replaced them when they seized up and fluid only flowed to one set of brakes. All 4 of mine work fine. Gush have a gutless soft as water peddle.

 

[Matthew Currie]

I wonder if a bad switch can not leak externally but leak internally, or trap a bubble in the switch guts or???? Mine does not leak on the outside. I have bleed my combo valve a got 3-4 dozen times while bleeding the entire system front to back and back to front.

Sealer? Maybe an anaerobic thread-locker when dry? Or an epoxy which is iffy. Might tray a fuel line thread sealer paste, industrial plumbing duty?

I would consider any brake part that requires thread sealing to be a goner by definition. All the sealing should be done at the double flare, and the only function of the thread is to push it tight. I would never put thread stuff on a brake line except perhaps to get home from the woods at night - for that anything is permitted.