Brake MC issue and questions Renix

I have been trying to solve the unsolvable problem of a mushy peddle when the engine is running and hard peddle when the engine is off. The MC is new (Autozone). I have a looong debugging thread that has been a work in progress for a very long time, but here is the short part of the current story, question today:

I have heard rumors that MCs should be bench bleed (like in a bench vise?) before mounting, but I have bleed mine while mounted on the vacuum booster the last 8 years or so with the little tubing kits and plastic holder that comes with the MC before I connected the brake lines. Bench bleeding and transport and mounting....a filled and bled MC seemed messier and more problematic than doing it mounted on the VacBoster to me.

Has anyone else had a problem doing this on a Renix jeep era MCs (or similar ones) and found air trapped in the MC by not bench bleeding first, versus bleeding while mounted on the VacBooster, and found the air bubble in the MC would stay there forever, with out a dismounted bench bleed? And old timer is telling me to pull it and bench bleed it, but I find it hard to believe that is the problem. I am thinking of re-bleeding at the MC using the vehicle tube fittings on the side of the MC. But if it is a rod travel distance issue will that work?

If that is not the issue, then my second question is has any one had a brand new MC that bleed fine and had a super hard peddle, normal feel with the engine off, and then the peddle turned to mush and nearly goes to the floor with the engine on at full vacuum in idle - park? The booster is only a year or two old and working very well, maybe too well, LOL.

My old thread is here showing that we have done every other conceivable thing multiple times!!!!


http://www.naxja.org/forum/showthread.php?t=1019972

My second question, is could the new MC have good enough seals, piston cups, to seal and hold pressure with no vacuum boost, but be leaking internally under the higher boost pressure? There are no external leaks, I have put 3000 miles on it with no loss of fluid.

Check out your old thread for my post.

Hmm, kinda surprised at the lack of replies here. Has no one had a bad out of the box, new brake master cylinder that acted like this?

Especially interested in answers to this?

"My second question, is could the new MC have good enough seals, piston cups, to seal and hold pressure with no vacuum boost, but be leaking internally under the higher boost pressure? There are no external leaks, I have put 3000 miles on it with no loss of fluid."

In other words I am trying to decide if it is a bad new MC out of the box (second one in 2 years) or an impossible to remove air bubble stuck in the MC itself.

I think the bleeding has bled itself to death. Not the problem.

I want to know the difference between the protrusion of the rod on the booster versus the depth of the cavity it protrudes into on the master cylinder.

cruiser54 said:

I think the bleeding has bled itself to death. Not the problem.

I want to know the difference between the protrusion of the rod on the booster versus the depth of the cavity it protrudes into on the master cylinder.

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I am going to check that this weekend. I verified today that MC on the 85, 87 and 89 are all the same. The first brand new cast iron Azone MC I installed (with a new Vac booster) was the 85, and it may be the one I recall adjusting the rod on, and not the 87. I did not bench bleed that one!!!! But it did take a while to bleed the wheels on it, and I never messed with the combo valve on it!!!

We jacked up the jeep and tested all 4 wheels tonight for any brake drag, and found very little if any. And we tested for how much brake peddle it took to lock up (turning the wheel by hand) using the brake peddle. It did not matter whether the vacuum boost was on or not, the front drivers side took only 1-1/4" of brake travel to grab, and the passenger side front took only 3/4" of brake travel to grab. I have decided the reason the passenger side is getting hotter is the brake pad is new (and flat) and the old rotor is not perfectly flat, it is slightly thicker near the center than at the outside, which is confirmed by the temperature profile. We are going to replace that rotor with a new one. But that is not causing the soft peddle.

I forgot to brake test the rears, peddle test while the jeep was jacked up, but we did check the adjusters and got the drivers side a little tighter, maybe 1.5 full turns, the drivers side was already perfect.

No change in the peddle feel yet.

I didn't read your old thread but you have new wheel cylinders and calipers installed I'm assuming?

Well, I better read up on your past posts to get apprised so I don't waste our time with such questions:sure:

I will post up if I can help:gee:

The reason you bench bleed is, the pistons for the front and rear brakes are multi chamber. Each piston has two seals and three chambers.

In my experience if one of the chambers in the master cylinder has air in it, it will eventually find it's way out. The reason a bench bleed is better is so one of the pistons doesn't collapse and form an air vacuum. If it collapses, for lack of a better word, it will likely never pump up and you will have no pedal.

Crazy stuff I've seen that may or may not be a factor, but something to check as a last resort.

A few new master cylinders I've installed had a casting flaws in the wall of the piston cylinder. This is more common on rebuilds, but can happen to brand new ones also. There is a pit in the walls of the master cylinder.

I actually had one, new one, somebody installed one of the pistons backwards.

A rubber brake line may have separated, they are layered and you may not see any obvious flaws, but one layer is for strength and when this separates the hose swells and can cause a soft pedal. This actually happens often, when you do the brakes, never let the front caliper hang from the hose, wire it up out of the way and don't kink the hose. I've had them slip and dropped the caliper when it hits the end of the hose, the weight of the caliper and the angle is almost guaranteed to kink the hose and may separate the layers or maybe even rupture the inner layer. A few things can happen, this can allow fluid between the layers and break the containment of the brake hose. If just the strength layer is broken the hose swells and you loose some pressure and/or the inner layer is soft and can act like a valve . I wouldn't recommend this on old hoses, but if you are careful not to overdo it, you can use a piece of cloth and a pair of vice gripes to clamp the hose shut near the top. If your pedal gets better you've found your problem. If you look closely you can sometimes watch the hose swell.

Remember your new shoes don't actually fit your old drums unless you get the drums turned and the shoes arched. New shoes in old drums and in all likelihood only a patch a couple of inches wide are actually hitting the drum. If the mismatch is fairly pronounced, the shoes can actually rock some.

I always sand my discs and drums with a hundred grit sand paper . Sand at an angle, first one way at a 45 angle to the rotation and then again at a 45 degree angle so you leave a cross pattern on your drum or disc. Sand the shoes and pads also. It helps them break in and seat properly, way faster.
It is in the book (Dodge truck) and the way Raybestos taught me in school. At least it is in the older books, one of those forgotten steps in a brake job that nobody seems to know anything about anymore. On some pads you have to sand them, they coat them in a preservative that may cause you to have little to no stopping (friction) the first few tries. The preservative is to keep the metal parts of the pads and shoes shinny and new looking, sometimes they are none to careful where the stuff ends up and it ends up on the friction surface also.


Drive in reverse, apply the parking brake then stomp on the foot pedal to get it completely stopped. Repeat 3-4 times. This really gets the self adjusters working and will tighten up those rears. A slight amount of drag on the rears won't hurt anything, it will wear off quick and help the curve of the shoes to fit the curve of the drums.

The rear wheel cylinders can also have pits or flaws in the cylinder walls. The easiest way to find then is to stick your finger in there and feel for them. Eventually some brake fluid will seep behind the dust cover on the wheel cylinder. The flaws can also let air in.

I'm out of ideas for the moment, I'll post up again if I think of anything else.

8MUD!!!! Thanks, your posts on topics like this are always helpful!!!!!

This one has been most odd. The last thing we tested was brake action on each wheel, jacking up the jeep, and testing for brake action on each wheel, engine off and on. It only took about 1" of peddle to make the front wheels too hard to turn by hand, about 1.5" to lock up the rear wheels. All the rubber hoses (including the rear one) except the passenger side are new and were inspected for any high pressure flaws (bubble while loaded) and as noted before, that side's brakes (Passenger side) are working the best of all 4, so that rubber hose has passed inspection (inside tests and outside visual load tests). We have excellent brake fluid flow to all wheels when bleeding.

I took it on the freeway yesterday, 80 mile round trip with no braking problems, in fact the brake power seems about as good as it ever was using just 1.5 to 2" of brake travel. The odd part is that once stopped, I can press a tiny bit more on the peddle and I get another 1-1.5" of peddle travel that fells like I stood on a bag of air!!!! Then it hits a wall, solid that feels like I am standing on concrete, just 1.5-2" off the floor. While bleeding, the foot peddle will travel ALL the WAY to the floor with no problem!!

While breaking, (say from 50 mph) that last 1.5" of travel gives me no additional breaking power near as I can tell!!!!

Also I can use about 65% of the soft peddle travel and nearly toss myself into the windshield now. If I hit the brakes hard enough I can lock up the fronts in forward now, and the rears in reverse. The crazy part is the last 35% of the soft foot peddle travel seems to do nothing. If I brake slowly (not all once hammer it to the bottom to lock the brakes up in a test), at first (say 1-2 seconds) and then push all the way as hard as I can, it does not lock up and the last 1-1.5" of peddle adds nothing to the brake power I already feel with just the first 1.5-2" of brake travel.

You comments have me thinking it might be a casting flaw deep into the cylinder wall, but not sure this is the symptom such a flaw would cause?

I still plan to try and check ( extend ) the booster rod length as Cruiser54 already suggested.

8MUD, can you explain this part any better?

In my experience if one of the chambers in the master cylinder has air in it, it will eventually find it's way out. The reason a bench bleed is better is so one of the pistons doesn't collapse and form an air vacuum. If it collapses, for lack of a better word, it will likely never pump up and you will have no pedal.

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The last thing we tested was brake action on each wheel, jacking up the jeep, and testing for brake action on each wheel, engine off and on. It only took about 1" of peddle to make the front wheels too hard to turn by hand, about 1.5" to lock up the rear wheels. All the rubber hoses (including the rear one) except the passenger side are new and were inspected for any high pressure flaws (bubble while loaded) and as noted before, that side's brakes (Passenger side) are working the best of all 4, so that rubber hose has passed inspection (inside tests and outside visual load tests). We have excellent brake fluid flow to all wheels when bleeding.

I took it on the freeway yesterday, 80 mile round trip with no braking problems, in fact the brake power seems about as good as it ever was using just 1.5 to 2" of brake travel. The odd part is that once stopped, I can press a tiny bit more on the peddle and I get another 1-1.5" of peddle travel that fells like I stood on a bag of air!!!! Then it hits a wall, solid that feels like I am standing on concrete, just 1.5-2" off the floor. While bleeding, the foot peddle will travel ALL the WAY to the floor with no problem, like it should!!

While breaking, (say from 50 mph) that last 1.5" of travel gives me no additional breaking power near as I can tell!!!!

Also I can use about 65% of the soft peddle travel and nearly toss myself into the windshield now (much better than before!!!). If I hit the brakes hard enough I can lock up the fronts in forward now, and the rears in reverse. The crazy part is the last 35% of the soft foot peddle travel seems to do nothing. If I brake slowly (not all once hammer it to the bottom to lock the brakes up in a test), at first (say 1-2 seconds) and then push all the way as hard as I can, it does not lock up and the last 1-1.5" of peddle adds nothing to the brake power I already feel with just the first 1.5-2" of brake travel.

It is almost like I get a normal peddle while driving, for the first 1 to 1.5" of travel, and then nothing but a soft spot, no additional braking the next 1.5" of peddle, then a hard wall 2" from the floor that also adds nothing to braking power. Pumping the peddle does seem to help some, but is not needed to lock the brakes. If I stomp on the brakes hard I can lock them up......

Ecomike said:

8MUD, can you explain this part any better?

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I did a really poor job of explaining that. A picture might help. This is the basic setup with a few minor variations for different manufacturers. You can see how air between the pistons can be hard to move and can effect volume. Bench bleeding or loop bleeding helps eliminate trapped air inside the master cylinder. You can push (collapse) the two pistons together and they can stick that way or separate very slowly, if some of the chambers have fluid (which doesn't compress) and other chambers have air or a mix of air and fluid. I better stop trying to explain this before I confuse myself completely.:doh:

What you called loop bleeding, I always do, I just started doing it on the vehicle, instead of a bench vise ages ago. The issue I was recently warned of is that mounted on the brake booster "loop" bleeding may not move the pistons the full length of the cylinder, and might leave a dead space of air at the end of the cylinder.

I must be getting close to fixing it, because the gremlins moved today to the engine, I have miss, at all speeds now :rattle:

Note from DJ
There is some information on this web site that may help.
How Master Cylinders and Combination Valves Work
Information from How Stuff Works
http://auto.howstuffworks.com/auto-parts/brakes/brake-types/master-brake.htm

Time for an update here. While I am still not sure if I have solved ALL the problems, I now seem to have the best brake action I have ever had on this jeep in 8 years (87 wagoneer) Last thing we did was replace the passenger side rotor (the one that was getting too hot and seemed to be doing most of the brake work as a result) and the driver side brake pads (so that both sides were brand new).

Most notably, I discovered that while the pass side rotor was not worn beyond OEM spec limits at the outside, it had a tapped wear pattern from outside to inside (on the OD of the rotor), and the inside wear was at the limit after all (substantial taper that was not obvious on visual inspection!). I verified it with a micrometer.

I caught it when I noticed that the new pad was only making contact at the outside and had a gap at the inside. Only about 30% of the pad was making contact!!! Years back the clips had been installed wrong and caused the brake pads ( about two sets of pads ago) to wear unevenly, and so it seems it had tapered the rotor(s) and it was NO LONGER FLAT and parallel.

I still plan to check into Cruiser54's idea. But the last 80 mile DD run, I started feeling like the brakes were normal finally, and better than any XJ Renix I have had. The peddle action is much closer to perfect now, if not perfect, after a few trips to brake in the new parts this week.

This is why you replace pads in sets and have your rotors turned or replaced.

cruiser54 said:

This is why you replace pads in sets and have your rotors turned or replaced.

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I have never, in nearly 1 million miles, about 40 brake jobs, all on my own junk, ever turned a rotor or worn one out, to the point where it affected the repairs and how well the brakes worked.

But I did discover 3 wrong ways to install the anti-rattle clips on the front of these jeeps!!! One way of which is what I think damaged these 2 front rotors. They visually looked flat still. Took the second micrometer test comparing the thickness across the diameter, and eyeballing the outer pad sideways under pressure to catch this.

Still trying to diagnose an Ecomike original brake problem on the 87 WagXJ since about 2 years ago.

All brake parts, and I mean all, are new since the problem arrived. With the engine off (no vacuum) I have a good hard, normal brake peddle.

With the engine running I have no external vacuum leaks, but the peddle is mushy for about 2/3rds of the travel to the floor and then it bottoms hard as a rock 1/3 of the way off the floor.

(I get good breaking for an old XJ at about 1" of travel, but little additional braking power the next 2 inches of travel and then it turns hard as rock with no more braking 1/3 of the way off the floor). It brakes much better at highway speeds than at 10 mph approaching a light or stop sign.

I have tried braking neutral with the engine revved up to raise the vacuum, but it makes no difference in neutral, while at freeway speeds in low gear the mushy peddle goes away.

I am out of ideas!!!

Note to self, pay more attention to 8Mud's suggestions like this one LOL.

8Mud said:

The rear wheel cylinders can also have pits or flaws in the cylinder walls. The easiest way to find then is to stick your finger in there and feel for them. Eventually some brake fluid will seep behind the dust cover on the wheel cylinder. The flaws can also let air in.

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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, installed about 2 years ago. 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 ...(IIRC) to replace the leaker, passenger side. Replaced them just to see if it made any difference.

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 internally with vacuum applied, but even that is hard to picture. After 4 NEW MCs (3 from AZ, one from Raybestos Rock Auto) on this rig and a 5th (from AZ) that worked perfectly on the white 4x4 jeep, I am done suspecting the MCs. My rubber hoses are all new, so have no reason to suspect them they passed a visual inspection with engine on and brakes applied at peak force..

Suspecting the Booster or continued wheel cylinder issues now.

I have used old unturned rotors and drums for 40 years with no problem except once, recently on a rotor. But this rig has new rotors and drums. Only part I have not replaced is the Combo valve and I see no signs it is at fault. Also have had no fluid leaks in 3 years.

I have had repeated problems getting air all out of the rear wheels!!! Now I know why!!!! In fact the latest problem was air at the new NAPA rear passenger wheel cylinder never ending.....

Why not go to a dual diaphragm booster like the wj booster and MC?

mtbxj87 said:

Why not go to a dual diaphragm booster like the wj booster and MC?

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I thought doing that would kill my squishy pedal (assuming my MC or booster were bad) and all it did was make my brakes stronger but still squishy. This is like the third thread on this same vehicle with the same issue so if you haven't read his other two threads, you won't have the whole back story. That's a great upgrade but I'd fix the existing issues before adding more variables. Hint: He's replaced both already with aftermarket OE compatible parts to no avail.

Things I noticed when reading the last few years worth of info on this. I'd highly recommend replacing left and right side parts at the same time. Lots of places I saw were one rotor, one wheel cylinder, one drum, one set of pads, etc were replaced at a time. It's just good practice to replace both sides together. The only thing that hasn't been replaced were the hard lines. Never know, one may have a clog or a pit in it that holds an air bubble even though the flow looks good at the bleeder.