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  #1  
Old December 12th, 2015, 22:56
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Ecomike Ecomike is offline
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Soft brake peddle cause

OK, tired of this 3 year battle with my 87 Jeep brakes

Tried three AZ (auto zone) new durajunk MCs, Master cylinders, new Vac booster, all new brake parts except the Combo valve, and I mean all new!!! No air in the system

With all three MCs, the brake peddle was rock hard with the engine off after 3 pumps of the peddle (first 2 had Vac still in the new booster).

And I mean rock hard!!

But turn the engine on (vac on) and the peddle feels like mush to the floor (one inch from the floor) in park, but drive it and the brakes work, I can slam on the peddle and toss someone into the dash at first, but they fad easy in a 4+ second freeway panic stop and press the peddle to the floor and the car coasts before stopping.

I now suspect a 3 year running batch of bad AZ MCs, tried 3, that are out of OEM spec on the piston cups and or cylinder wall diameter on these new MCs, and thus can not holder pressures when the new vac booster is on?

Any counter ideas or does everyone agree I need a new source for Renix era MCs?

Any suggested new source or brand? I want cast iron, had too many 1 year old failures with corroded (not properly anodized) aluminum ones in the prior 6-8 years from AZ.

Thinking Rock auto, but what brand? Any heavy duty aftermarket ones???
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091

Last edited by Ecomike; December 12th, 2015 at 22:59.
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  #2  
Old December 13th, 2015, 00:45
mtbxj87 mtbxj87 is offline
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Re: Soft brake peddle cause

MC could be bypassing. Drums adjusted, rotors and drums not glazed? Vacuum leak somewhere? I would try rebuilding a used original before I tried a autozone MC
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  #3  
Old December 13th, 2015, 08:45
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Re: Soft brake peddle cause

Could it be a brake line? Has more pressure with motor running doesn't it (Power brakes)?
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Old December 13th, 2015, 08:57
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Re: Soft brake peddle cause

Worn out brake lines, poor drum adjustment, even low quality pads & worn rotors can all cause what you are describing. I don't really understand why pads & rotors would cause it but I've seen it & heard it enough times to include it on the list.
Personally, I like Centric rotors & Fleet Service pads. The para-aramid (kevlar) pads make a lot of dust but they flat out work, don't fade, and pedal feel is better than with other pads I've tried
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Old December 13th, 2015, 09:07
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Re: Soft brake peddle cause

Do you loose brake fluid? Is the pedal rod to booster rod length correct? Also, is the booster to master cylinder rod length correct?

The master cylinder/s could have internal leaks, i.e. the fluid could be bypassing the seals and going back into the fluid reservoir. That would be my first and only guess.

I hate to say this but I do not purchase parts for my vehicles from Auto Zone. Oriley's or Napa are the only places I buy from. They are a bit more expensive than Auto Zone but I almost never have to do the same thing twice.
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Old December 13th, 2015, 12:53
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Re: Soft brake peddle cause

Front calipers installed on the wrong sides; bleeders on the bottom and not the top?

It can happen to the best of us.

It also result in your exact symptoms.

I know these things.
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  #7  
Old December 13th, 2015, 14:02
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Re: Soft brake peddle cause

Quote:
Originally Posted by mtbxj87 View Post
MC could be bypassing. Drums adjusted, rotors and drums not glazed? Vacuum leak somewhere? I would try rebuilding a used original before I tried a autozone MC
Please explain in detail "MC could be bypassing"

All the usual suspects have been covered, ALL and I mean ALL new parts tried repeatedly for 3 years never made any difference. Been doing brakes for 40 years, never had this problem before.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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Old December 13th, 2015, 14:05
mtbxj87 mtbxj87 is offline
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Soft brake peddle cause

Ok, I have no doubt you know what your doing, I didn't imply that. Basically what techno1154 said above. Fluid is passing by the first piston in to the next
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  #9  
Old December 13th, 2015, 14:09
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Re: Soft brake peddle cause

Quote:
Originally Posted by scottmcneal View Post
Could it be a brake line? Has more pressure with motor running doesn't it (Power brakes)?
Replaced 100% of the brake lines, full flow to all calipers and wheel cylinders, bleeds easy, no air left anywhere.

yes it is power brakes, that is what a vacuum booster does, it multiplies the area of on piston over the over increasing the brake fluid pressure in proportion to the ratios of the area, Vac booster diaphragm to MC piston area and the vacuum uses atmospheric pressure to boost the foot applied pressure...

Yes more pressure (thus more brake power) is applied to the MC with the engine on, vacuum on, thus if the seals are marginal they could leak internally at the higher pressure, and hold at lower pressure, is my only idea as to what the problem could be. Hold say at 500-1000 lbs (foot pressure only, no vacuum boost), and leak at say 2000 to 5000 lbs with the vacuum boost on.

There is NO vacuum leak at all.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #10  
Old December 13th, 2015, 14:15
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Re: Soft brake peddle cause

Sorry about the question on power brakes, didn't notice that. My bad..
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Smiley face added so people won't feel but t hurt.
Don't make old people mad.We don't like being old in the first place, so it doesn't take much to piss us off.
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  #11  
Old December 13th, 2015, 14:21
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Re: Soft brake peddle cause

Quote:
Originally Posted by yossarian19 View Post
Worn out brake lines, poor drum adjustment, even low quality pads & worn rotors can all cause what you are describing. I don't really understand why pads & rotors would cause it but I've seen it & heard it enough times to include it on the list.
Personally, I like Centric rotors & Fleet Service pads. The para-aramid (kevlar) pads make a lot of dust but they flat out work, don't fade, and pedal feel is better than with other pads I've tried
But that kind of fade would not cause a soft peddle that goes to the floor. I get more braking at 1/2 peddle than I do if I press to the floor-bottom as hard as I can.

I have solved brake fade, weak brake action with new parts before that were caused by tapered rotors, for prior pads that lost or damaged rattle clips making the pads work at a bad angle. The taper was found using an IR gauge that showed 300 F on the top of the rotor and 130 F on the lower area. Confirmed with a machinists caliper that the top of the rotor was in spec, the lower ws worn past the OEM wear limit. All the contact was at the outer extreme of the pads. new rotors made a huge difference. Also had caliper missaligned before, bad caliper bolts and so on.

In this case I have replaced and verified all parts several times over 3 years now and never got to the source of the particular problem.
Last bad part problem was a crazy one to find. Kept getting air in the rear brake lines, with no fluid leaks. Turned out the rear drivers side wheel cylinder (only 2 year old AZ part) was letting air in, but did not leak fluid. One brake mechanic confirmed that this does happen, rare but possible. First new AZ wheel cylinder I installed last month leaked like crazy just trying to bleed it. Switch brands finally to O'Reilly. Got two good ones, no problems so far from them.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #12  
Old December 13th, 2015, 14:28
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87manche 87manche is offline
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Re: Soft brake peddle cause

I have found autozone brake hydraulic parts to suck balls.

rockauto has centric and raybestos parts for reasonable pricing. if it's not an absolute emergency that's what I go with. No problems.
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Old December 13th, 2015, 14:32
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Re: Soft brake peddle cause

"Do you loose brake fluid?"

NO, no leaks in 3 years.

"Is the pedal rod to booster rod length correct?"

Yes, never changed it.

" is the booster to master cylinder rod length correct? "

Tried increasing the length as far as possible 3 years ago, it did not solve the current 3 year old problem, but helped some at the time.

"The master cylinder/s could have internal leaks, i.e. the fluid could be bypassing the seals and going back into the fluid reservoir. That would be my first and only guess. "

That is what I now suspect, but I never had one do this before, never had a hard peddle engine off, super soft engine on. And not on 3 brand new MCs in a row. Soft peddle on both from a failed MC yes.

And before anyone asks we bench bleed the pioties out of all 3 MCs.

I never had problems with AZ parts 20 years ago, or 10 years ago, but the failure rate the last 3 years is near 50-70%. Just crazy. Most of my parts on my 5 jeeps are about 70% AZ now, lifetime warranty, but looks like that is changing now.

So if I go for a 4th new MC from rock auto, which brand(s) is best?

Quote:
Originally Posted by techno1154 View Post
Do you loose brake fluid? Is the pedal rod to booster rod length correct? Also, is the booster to master cylinder rod length correct?

The master cylinder/s could have internal leaks, i.e. the fluid could be bypassing the seals and going back into the fluid reservoir. That would be my first and only guess.

I hate to say this but I do not purchase parts for my vehicles from Auto Zone. Oriley's or Napa are the only places I buy from. They are a bit more expensive than Auto Zone but I almost never have to do the same thing twice.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #14  
Old December 13th, 2015, 14:39
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Re: Soft brake peddle cause

Quote:
Originally Posted by Bent View Post
Front calipers installed on the wrong sides; bleeders on the bottom and not the top?

It can happen to the best of us.

It also result in your exact symptoms.

I know these things.
Good to add to the list, heard of this before, but it is not the problem on this rig at this time LOL. We checked.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #15  
Old December 13th, 2015, 14:48
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Re: Soft brake peddle cause

Quote:
Originally Posted by 87manche View Post
I have found autozone brake hydraulic parts to suck balls.

rockauto has centric and raybestos parts for reasonable pricing. if it's not an absolute emergency that's what I go with. No problems.
Well if it is three bad MCs in 3 years, at least Durajunk is consistantly a POS-junk

I'm in no rush, been fighting this problem for 3 years on that rig, marginal mushy brakes. But with my eyesight going and insane Hollywood wanna be stunt drivers on the freeways here, I need better brakes now. 4-5 seconds of space just encourages these idiots to jump on my front bumper in a dangerous lane change, then if they hit their brakes I nearly land in their back seats before I can get enough braking to stop or back off their bumper. Never been a tailgater, so I've been able to get away with marginal brakes, but today's cars brake so much faster than these jeeps, the jeeps need perfect brakes on a DD.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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