Soft brake peddle cause

[Ecomike]

Update, using 8Muds hose clamp diagnostic test, I got mush for the peddle after closing off the passeneger side hose (the one with a new caliper and hose, that this test showed had a bad caliper or hose). Blead all the brakes again, and retested, same thing. Then tried clamping off the drivers side hose and got a hard peddle, so bled that side again, then clamping off that hose again got mush again? The hose clamp test is not giving consistent results. Also noticed that in one clamp test the wheel would no longer turn, in the other clamp tests the wheel did turn? Both hoses are new. Scratching my head again. I have good brakes, but a deep stroke and mushy peddle still. Maybe it needs new muffler bearings?

WE do have solid wheel lock up off the ground on all 4 wheels with about 1" of brake peddle travel, and I can lock up 2-3 wheels, maybe 4?, and skid the jeep in a panic stop if I go heavy and hard enough on the peddle now.

So this weekend we clamped off the passenger side brake hose, and bingo we had a solid peddle with engine on, vacuum to the booster on.

I had already procured 2 new rotors in desperation to try and we replaced the front rotors on both sides and got no change in the brake peddle mush.

So we ran the hose test and got a confirmation the passenger side front was the source of the mush per 8muds suggestions.

So I procured a rebuilt duralast (LOL) caliper and new hose. Bled that caliper and the combo valve ONLY. Still mush?????

But now I have great brakes, it just takes a 3-4" stroke to the floor to get there now?????

Now I am really puzzled!!!!

I plan to bleed the entire system again tomorrow, but I figured the rears would not get air (combo valve block it to the front?) , but perhaps that is wrong?

Any thoughts front the gallery here about using a real vacuum pump and a spare MC lid to make a vacuum system to bleed (suck) the air out from the top side all at once???? Can the closed system handle that much vacuum???? Or should I use a limited vacuum like the el-cheapo ones that Harbor freight sells. I can supply nearly 30" of hard vacuum with my large lab vac pumps, or my AC professional vacuum pump, but they might suck the seals out the calipers or wheel cylinders unless they are designed to handle it??????

 

[Ecomike]

I have all new parts including master cylinder, booster, calipers, rear cylinders and hardware, shoes, pads, front brake lines. I used the hand vacuum pump and pumped at least 2 quarts through the system, starting from the furthest cylinder to the closest caliper. I have a solid pedal until I start the engine, then the pedal goes to the floor. The wheel bearing does make sense, but the car had not moved. Stumped I am, and it takes a lot to stump me.

I am wondering if the aftermarket parts we are getting are mismatched now on the vacuum booster over MC pressures? I wonder if there are not a lot more with this same problem that have given up and switched to newer double diaphragm upgrades?

 

[Ecomike]

That is the part I have never had before also, that has me so puzzled

"I'm just confused on why I have a firm pedal until the engine runs."

until this one wagoneer. Even the 89 cherokee that had brake fade had a hard peddle with the engine running. The brake fade was a tappered rotor that was in spec on the outer area and worn too much at the center that I missed in my micrometer inspection the first time. I found it using an infrared temp sensor after a hard drive and confirmed with the micrometer.

I still suspect that no one has a good cast iron MC on the shelf anymore for the 87 jeeps? And the last aluminum ones I got corroded in under 12 months due to poor anodizing.

I failed to mention that I have a rebuilt mc and a new mc from napa, and both are bench bled and both give me the same results. Rear brake shoes have been adjusted and re-adjusted. I adjusted the emergency brakes and double checked Everything. I also have a '86 Cherokee with a v6 with brakes that I can stop on a dime, so want this '89 to be the same. My next attempt is to reverse injection my brake lines. I'm just confused on why I have a firm pedal until the engine runs.

 

[Ecomike]

I am looking forward to your report on that!!!! I can't see how the Prop valve could be a problem with the engine running and not with the engine off!!!! I am begging to believe the Vacuum booster pressure is just a little too much for these recent MCs, that are either over-bored or undersized on the piston cup parts? But how to solve that problem? I have 4 brand new MCs doing the same thing.

The proportioning valve is the only item in this system I have not dealt with, but usually they do not fail. Or am I wrong? I understand how it works, so it makes me think that the problem is at the front, because of the rears locking up. I am hoping that if the is a pocket of air in the valve, the reverse injection bleeding will help. I have to try this before I go any further, and I will post here my results, good or bad. Thanks for your help.

 

[Ecomike]

Well, since the proportioning valve is the only part I haven't changed, I will be changing it out before I do a reverse injection bleeding. Thank you, and I will let you know my results. Cheers.

I look forward to hearing the results of that as well. It is the only thing I have not tried, but from what I have read and my test results it is not the prop valve in my case that is the problem. I am tempted to block off the vacuum line to my booster and test the brakes next!!!!

 

[dan1977p]

I look forward to hearing the results of that as well. It is the only thing I have not tried, but from what I have read and my test results it is not the prop valve in my case that is the problem. I am tempted to block off the vacuum line to my booster and test the brakes next!!!!

If there is air in the top front corner of the combo valve, how will this help vs normal bleeding procedures? The fluid will still go the easiest route and bypass that bubble so it won't get picked up and flushed out. Try leveling the combo valve and make sure no brake lines have loops in them that could also trap air bubbles and be difficult to get out.

This all assumes the pads and shoes are making proper contact with their rotors and drums, which you have said has been a problem multiple times with this vehicle. This is also assuming we aren't continuing to blame the hardware. I get that there are lemons out there so getting one is a possibility, getting two is a rarity and getting three or more is downright inconceivable. At that point, the entire batch is probably bad and there would be mass returns from multiple customers who are bought MC's out of that batch.

So, before continuing to flush the system, have you confirmed that pad/shoe contact is correct? Both sides pads/shoes/rotors and drums have even thickness from left to right? Both drums hardware is new and adjusted the same? Both soft lines on the front are the same age and if one was replaced, was the other? If not, replace both. In reading the history of this thread, replacing things in pairs was not done, and though technically possible, it's not a good idea.

I wouldn't touch the booster since it's obviously working. If it wasn't, it wouldn't be adding enough pressure to send the pedal to the floor so you would get the hard pedal you're searching for but really poor brake performance.

 

[davesdetail]

I am wondering if the aftermarket parts we are getting are mismatched now on the vacuum booster over MC pressures? I wonder if there are not a lot more with this same problem that have given up and switched to newer double diaphragm upgrades?

The booster sends more power to the master cylinder, so I'm not yet convinced that the booster is a issue. And, because my '86 works great with the single diaphragm, I don't think I need to upgrade.

 

[davesdetail]

That is the part I have never had before also, that has me so puzzled

"I'm just confused on why I have a firm pedal until the engine runs."


I'm thinking it's down the line somewhere. Is it possible that the new brake lines on the front are made of a low pressure hose, causing them to bulge out when the engine runs? I wouldn't think so, but I'm grasping at straws here.

 

[Ecomike]

One thing that did help me was bleeding the combo valve first and last. I had never had to that on the other jeeps.

 

[Ecomike]

The booster sends more power to the master cylinder, so I'm not yet convinced that the booster is a issue. And, because my '86 works great with the single diaphragm, I don't think I need to upgrade.

But what if the booster is putting out too much pressure on the MC and causing the MC valves to bypass internally?

 

[garr]

I would do a good power bleed with the rear end jacked up high enough to level out the master cylinder to ensure that there is absolutely no air anywhere in the system before messing with anything else. I did this to a completely new system (new hard/soft lines, calipers, MC, everything replaced) last week for a buddy, amazing how much air can come out of the system.

 

[davesdetail]

That is possible, although when I bench bleed, I am only able to put so much pressure on the mc, and when the engine is off, I can stand on the brake pedal and it stays solid. That's why I'm thinking that there is a weak point down the line somewhere, like a weak brake line or lack of contact at the calipers with the discs. I'm still stumped, but I will be at it by Monday.

But what if the booster is putting out too much pressure on the MC and causing the MC valves to bypass internally?

 

[davesdetail]

Thank you, and this does make sense. I now have a reverse injection bleeder system that I have yet to try. It forces brake fluid from the cylinders/calipers up to master cylinder, letting air rise up. I'm thinking that I should have the front end higher with this procedure ??
Google: Phoenix reverse fluid injection clutch & brake bleeder.
Thanks again. atriot:

I would do a good power bleed with the rear end jacked up high enough to level out the master cylinder to ensure that there is absolutely no air anywhere in the system before messing with anything else. I did this to a completely new system (new hard/soft lines, calipers, MC, everything replaced) last week for a buddy, amazing how much air can come out of the system.

 

[davesdetail]

Okay, so I am still stumped! If I do have air in the system, why would I have a solid pedal with the engine off?

Ecomike, how can too much pressure from the booster bypass internal valves in the master, when I can not bypass them with a heavy foot when the engine is off?

 

[Ecomike]

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.

 

[Ecomike]

Okay, so I am still stumped! If I do have air in the system, why would I have a solid pedal with the engine off?

Ecomike, how can too much pressure from the booster bypass internal valves in the master, when I can not bypass them with a heavy foot when the engine is off?

With the vacuum off the brake fluid pressure may only be a few hundred PSIG, with vacuum applied it is likely several 1000 pisg. I do not know the exact pressures.

 

[davesdetail]

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?

 

[Ecomike]

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?

Depends on what the fluid is and where it came from.

Answer, maybe.

I have not found any signs of Combo valve leaks on mine and I always bleed the combo valve when I bleed anything.

 

[Ecomike]

I ran my no Vac Booster vacuum test today. Clamped off the vacuum hose tight. Peddle was hard but had nearly zero braking power. Unclamped I had a soft peddle and almost decent braking action back. Still no new clues. Any one know what the AMC-jeep XJ brake fluid line operating pressure specs are?

 

[garr]

I'm thinking that I should have the front end higher with this procedure ??
atriot:

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.

. I did notice on the proportioning valve some fluid under the connector on top, could this be a issue?

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