Soft brake peddle cause

[Digger87xj]

I only skimmed, so forgive me if it's been asked..

You are bench bleeding the master before installing it, ya?

 

[8Mud]

Just hypothesizing here, but a few things I can think of that my be a factor.

One is low vacuum, the booster is kind of a balancing act, it helps reduce the brake pedal pressure needed, but is a balancing act. To much help and the booster could theoretically apply the brakes on it's own. To little help and you need more foot pressure on the brakes. Low vacuum might be an issue. Might be a bad valve in the motor, a vacuum gauge needle can pulsate when a valve is bad. When it pulsates you loose a lot of vacuum volume.

Initial adjustment on the brake pedal rod, length?

The vacuum booster check valve is worth a look.

The rubber diaphragm in the master cylinder cover. It should expand enough to displace most of the air on the fluid side. If not it is possible for air to mix with the fluid (tiny bubbles) as the fluid agitates from all the vehicle motion.

More likely you have a tiny seep someplace if you are getting air into the system. The brake fluid in the lines and cylinders actually has a momentary slight vacuum when the brake pedal is released. Think of the difference in viscosity between the fluid and the air.

You may have more than one issue.

New pads and/or shoes. It takes awhile for them to wear in. Especially the rear shoes. The difference between the diameter of the drums and the curve of the shoes can be pronounced. Same with the front pads. There almost has to be some mechanical play in there. The rear shoes wear in really slowly.

On high performance brake systems I always matched the diameter of the drums with the curve of the shoes. Measured the diameter of the drums with a brake vernier caliper and arched the shoes with a shoe arch grinder, to make them a matched set.

Most of the pedal play is usually in the rear shoes in my experience. Maybe a design flaw, maybe the play is necessary to deal with expansion when everything gets hot.

Been a long time since Raybestos sent me to school to learn brakes, the basics really haven't changed much. Just a side note, but automotive isn't all they do, everything from planes and trains to tiny electric motors have brakes.

Every XJ I've ever had seemed to have a couple of inches of pedal play, sometimes more. Never seemed to hurt anything.

You might want to investigate how the booster works, may be interesting. The differential between the atmospheric pressure side and the vacuum side of the booster diaphram, pounds per square inch times (X) the surface area of the vacuum boosters diaphragm. In a nutshell. Dual diaphragm booster is just a way to increase the surface area and not increasing the diameter.

 

[8Mud]

Check your rubber brake lines for expansion. A little expansion is normal, but the lines are layered. When one of the inner layers separates or ruptures there may be little to see from the outside. Maybe a slight bulge that is hardly noticeable. I have better luck finding the bulge while having some one slowly pump the brakes and feeling with my finger tips. I can feel things I may not see.

Kinking the front brake lines during a brake service is common and the most common way to have the layers separate in the your rubber brake lines. Never let the calipers hang from the rubber brake lines while doing a service. Wire the calipers up out of the way, avoid kinking the rubber brake lines. One of those "oh chit" moments.

Tie your wire up to the top of the shock (or wherever) before you remove the caliper bolts. Or be prepared to look like a one armed wallpaper hanger.

 

[bchulett]

I chased brake issues on my 1991 for years. Swapped out every component that made sense.. even installed braided lines and the pedal would still would go to the floor.

Flushed the lines... pressure bled the lines... you name it, I did it.

Then one day, I had it at the dealer to get a few things fixed.. told them to take a look at the brakes.

A few hours later I got a call saying I had a rear wheel cylinder leaking.

You don't know how many times I looked at the rears, with no sign of a wheel cylinder leak - but after they installed a new one my five year, piss-poor braking ordeal disappeared.

YMMV.

.

 

[bchulett]

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.

I swear the more I read this the more it sounds like exactly what I went through. I was ready to throw more money at it - like installing a later model dual diaphragm MC and all the brake mods I could find. Now the brakes are as good as they were in 1991 when my Jeep was new.

Mine is lifted 5" with 31's... The brakes were so bad (even after I changed everything) I wouldn't allow anyone to drive it. I put a hault on increasing the tire size until I found a fix. I replaced everything except for the rear backing plates, wheel cylinders and pre-formed brake lines - and it cost me a lot of money.

I could have installed rear disk conversion kit and dual piston MC for the money I spent chasing this brake issue... and I've been doing brakes since the 60's. I've experienced brakes sucking air into the system before - specifically front single piston GM calipers on two different vehicles.

Before going any further I would swap out the front and rear rubber brake lines with stainless steel braided lines. It does make a difference with pedal pressure. The next thing I'd do is install Mopar Value Line brake pads and rear shoes - get away from the aftermarket.

I think you may be ingesting air somewhere in the hydraulic lines ... perhaps the rear cylinders or front calipers.

Did you rebuild your calipers and wheel cylinders yourself?

.

 

[Ecomike]

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.

Thanks!!! I may remove the rear drums and have a machine shop mic them top to bottom and make sure they are good front to rear. But I locked up the rears for the first time in a light panic stop today with no problem (did not even take the peddle all the way down). The adjusters are adjusted and working (A rare thing for old jeeps, installed new adjusters LOL). I did discover that the wear limit specs on a rear drum is a fraction of the new shoes wear material thickness. I would suspect more bad wheel cylinders at this stage, maybe letting air in.

This comment

"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. "

may be very helpful!!! If I clamp off the rear hose 100% and the soft peddle is gone it isolates the problem to the rear!!!! 8muds idea sounds great now!!

 

[Ecomike]

I only skimmed, so forgive me if it's been asked..

You are bench bleeding the master before installing it, ya?

Yes, but I never had to bench bleed before in 40 years on many vehicles.

 

[Ecomike]

I chased brake issues on my 1991 for years. Swapped out every component that made sense.. even installed braided lines and the pedal would still would go to the floor.

Flushed the lines... pressure bled the lines... you name it, I did it.

Then one day, I had it at the dealer to get a few things fixed.. told them to take a look at the brakes.

A few hours later I got a call saying I had a rear wheel cylinder leaking.

You don't know how many times I looked at the rears, with no sign of a wheel cylinder leak - but after they installed a new one my five year, piss-poor braking ordeal disappeared.

YMMV.

.

I am already suspecting the new wheel cylinders since one was bad out of the box this time. The leak was huge and obvious, and one from the install 3 years ago was it seems the source of air getting into the lines, with no hydraulic leaks visible or showing up in the MC reservoir over time.

 

[Ecomike]

I've experienced brakes sucking air into the system before - specifically front single piston GM calipers on two different vehicles.

Very interesting. Are the Jeep calipers GM?

I think you may be ingesting air somewhere in the hydraulic lines ... perhaps the rear cylinders or front calipers.

No doubt. It IS ingesting air in the rear section.

Did you rebuild your calipers and wheel cylinders yourself?

.

No, gave up on that decades ago. the 70s, Waste of time and money

 

[Ecomike]

8mud, I have pretty much exhausted your list already. Down to suspecting a bad wheel cylinder, again, now, or caliper but the air problem keeps showing up on the rear lines. The hoses are new.

 

[bchulett]

Originally Posted by bchulett View Post
I've experienced brakes sucking air into the system before - specifically front single piston GM calipers on two different vehicles.

Very interesting. Are the Jeep calipers GM?

I think you may be ingesting air somewhere in the hydraulic lines ... perhaps the rear cylinders or front calipers.

No doubt. It IS ingesting air in the rear section.

I don't know if they're GM... but here's the story how I found this can happen.

It happened on both my '69 Chevelle and '69 442 Oldsmobile.. same unibody and brakes shared between platforms. When I was doing the brakes I pushed the pistons back into the calipers... this is acceptable, right? Nope.. wrong!

The fact is when you do this fine silt that accumulates behind the boot can score the o-ring that's encased in the caliper.. The o-ring seals the piston, like the piston ring seals the compression in a combustion chamber. Well what I found was it didn't leak, but it sucked air into the system after bleeding the brakes.

It would take about a week before finding my brake pedal was touching the floorboard.. with no leaks whatsoever. This was a stark revelation... from that moment forward I would always rebuild the calipers.

But that was then and this is now... now I just buy new calipers.

.

 

[Ecomike]

Will try clamps on the hoses, 1 x 1, next as an isolation tool.

 

[Ecomike]

Will try clamps on the hoses next, 1 x 1, next as an isolation tool.

 

[Ecomike]

Will try clamps on the hoses next, 1 x 1, next as an isolation tool.

Houston!!!! We have brakes!!! Well at least working well enough now to drive the beast safely.

I can now toss unruly back seat drives through the windshield using the brakes LOL

Not perfect yet, still a soft peddle, but not as soft and solid rear brakes now. Tried a new one man brake bleeder kit and some new techniques on bleeding the rears and got some more air out and some solid brake action now in forward and reverse. Did not try the hose clamp yet. Will re-bleed the fronts again next.

Now if I can just figure out to swap my too hard to push clutch peddle for the too soft brake peddle LOL.

 

[Ecomike]

I should have quit while I was ahead. The perfectionist in me is not always so great I guess.

Took another stab at the 87 brakes. Used a clamp to cut off the flow to the rear brakes clamping the rear flex-rubber hose off. No change in the soft peddle, SO the rear brakes are not the problem, or not the only problem if there is more than one problem. Also the rear brakes are finally air free, yeah!!!!

So I put brand new rotors on the front just for grins and the brakes now make a racket up front with heavy pushes on the peddle and almost no brakes till I push hard on the peddle (brake action is OK when the racket starts which is when I stand on the brake peddle after the soft peddle range is used up) ?????WTF???? Will check the caliper bolts and rattle clips when it warms up.
This is the toughest dang gremlin I have ever chased.

I should have stuck with the mushy peddle and good brake action while I was ahead I guess.

 

[garr]

What condition are the solid lines in?
Particularly the line going to the rear that looks like it was mounted under salt water in every XJ I've seen here in NY!
Gotta wonder why Chrysler didn't spend the extra pennies for stainless on such an important line in such a vulnerable to rust spot!

The reason I ask is because I've seen small SWEATING like leaks on this line that are hard to detect.

 

[Ecomike]

This XJ was born and raised in a no rust zone, Houston Texas. The steel lines look new even after 30 years.

 

[Ecomike]

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??????

 

[Digger87xj]

are you starting your bleeding process at the RR, continuing - LR- RF- LF then bleeding the lines at the master?

Don't pump the peddle... Use the one stroke method; Open the bleeder, have someone push the peddle to the floor, close the bleeder, let up on the peddle, count to 10, repeat. This will move a massive amount of fluid and will remove all air. In 28 years as an auto tech, this is the only method I use and it works perfectly every time.

 

[MT Mike]

You have listed numerous failures with Autozone parts. Don't you have a NAPA or CarQuest near you? Almost every automotive shop I have ever dealt with will only use parts from those two parts houses.
It is time to buy better quality parts.

Also, Digger87xj's bleeding process is the only one I have ever used, and I have never had it NOT work.