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Soft brake peddle cause

Ecomike

NAXJA# 2091
NAXJA Member
Location
MilkyWay Galaxy
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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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
 
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
 
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.
 
Front calipers installed on the wrong sides; bleeders on the bottom and not the top?

It can happen to the best of us. :doh:

It also result in your exact symptoms. :D

I know these things.
 
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.
 
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
 
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.
 
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.
 
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.
 
"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?

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

It can happen to the best of us. :doh:

It also result in your exact symptoms. :D

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.
 
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.
 
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.

Looks like Rock auto is cheaper that AZ on the MCs today, no core charge and decent freight. I am ordering the cast iron RAYBESTOS MC39419, for $32 :eek: plus $8 freight. Not bad . Wish me luck folks!!!
 
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.

I just remembered I bought a 4th MC from AZ, used it on the other 4x4 white jeep I bought 3 years ago that had no brakes at all. That MC worked perfectly ( I think, at least in the driveway and short test drives it did, before the fuel pump, battery and front bearings died....). What are the odds that three of 4 were bad and ended up on the same jeep?
 
I changed my brake Master cylinder (87 Wagoneer) to a Raybestos brand from Rock auto, but while it seems a little better than the 3 new AZ duracrap ones I tried the last 3 years on this rig, I still have a mushy peddle with the engine running, and solid peddle with the engine off. Mushy all the way to 3/4s of the way to the floor, then it is rock hard. All new parts front and back, new rotors, new drums, new pads, new hoses, new hardware, new wheel cylinders, ...

and a new Vac Booster (3 years ago, about the time this started). This one is one of the worst problems I have ever had to debug. Did find bad (new and recently new) wheel cylinders from AZ. One was not leaking fluid, but was letting air in. Never seen that before, but I hear it does happen. One new AZ wheel cylinder leaked fluid right out of the box this month. Switched brands and sources to Napa.

Still wondering if there is a rare fault with Vac boosters that can cause this problem, but so far that makes no sense to me unless some parts of the booster are collapsing with vacuum applied, but even that is hard to picture.
 
if the booster fails the pedal just gets hard. I've never had one that caused a mushy pedal.

Not saying it' isn't possible, I've just never had it happen or heard of it.

since it stays mushy until the bottom I'd strongly suspect an issue with the rear circuit, since that's what generates the majority of the pedal feel.
Are you sure the rear brakes are working and adjusted properly? Lots of rear shoe movement will do that too.
I wonder if your proportioning valve might be wonky too.
 
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