Soft brake peddle cause

[davesdetail]

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.

 

[Green Mesa XJ]

Just went through something very similar . Power booster and master cylinder were replaced. Took days of fiddling with the brake pedal feeling like it goes to the floor , but felt good with engine off. Had a lot of air still bleeding out. I think part of it was bleeding the mc, the the new lines and open lines that let air in. Bleed and bleed the fluid. At least a quart and a half.

By that time it's as you describe. Felt good engine off and like crap with the engine running.
Ended up taking the rear drums off and manually adjusting them. After that it felt a lot better although I've never been happy with the way the old Xj brakes felt. My 85 Xj has a ford 8.8 with disc brake now that one feels decent.

 

[davesdetail]

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.

 

[davesdetail]

I am interested in the outcome of the OP ekomike and his issue.

 

[Green Mesa XJ]

My rebuilt Mc, booster combo was supposed to be pre-bleed as well. It seemed to take forever to get any real feeling back to the pedal. Then adjusting the back brakes to feel somewhat normal.

How did you brakes feel before? Mine (92xj) have always felt sort of soft.

Edit : the same thing puzzled me because before the motor turned on the brakes felt incredibly solid. And everything was new or nearly new.

 

[davesdetail]

I had good brakes before, but the pedal would slowly go to the floor one day, so I knew I had a Master Cylinder issue, and when I checked the condition of the pads and shoes, I had to go all the way, but I have failed to get them back to where they were.

 

[davesdetail]

No leaks, and nothing anywhere is wet.

 

[davesdetail]

Also, with the pedal going to the floor on a test drive, the rears are able to lock up.

 

[Green Mesa XJ]

I am interested in the outcome of the OP ekomike and his issue.

X2 . I'm following this thread hoping to find out what the cause is.

I was in the same boat as you, puzzled why it was solid before the engine turned on and the felt like a death trap running.

As I was trying to figure it out I was almost certain it was isolated to the rear brakes, or part of the mc dealing with the rear. proportioning valve came to mind as well.

 

[Green Mesa XJ]

Also, with the pedal going to the floor on a test drive, the rears are able to lock up.

Adjusted ours take only a little pressure to start the braking process if it's pushed some what hard it really slams to a stop. Even before adjusting the back I could hear and feel the brakes (front and back) engaging and releasing.

I know it sucks to get advice you've tried already with no result. I'm convinced there has to be a better reason for these brake problems. Also not competitive happy with how ours feels even though it seems to be working somewhat satisfactorily now.

I don't get why the ancient AMC XJ 's with lesser brake calipers and drum size has always felt better braking.

 

[davesdetail]

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.

 

[davesdetail]

My other problem I have is that I Refuse to take it to a mechanic! I am very mechanically inclined and I have done my own brakes on every vehicle I owned since I was 16, from passenger cars to 3/4 and 1 ton trucks. I don't like to be stumped!

 

[Green Mesa XJ]

My other problem I have is that I Refuse to take it to a mechanic! I am very mechanically inclined and I have done my own brakes on every vehicle I owned since I was 16, from passenger cars to 3/4 and 1 ton trucks. I don't like to be stumped!

I know the feeling. Once there is a problem I like to know the answer. Sort of a Rubik's complex.

started out having to work on my own cars, now I hate taking them in for anything (also I already bought all those tools) . As a kid I was fixing/ taking apart every machine I could get my hands on cars were the next natural progression.

 

[davesdetail]

I know the feeling. Once there is a problem I like to know the answer. Sort of a Rubik's complex.

started out having to work on my own cars, now I hate taking them in for anything (also I already bought all those tools) . As a kid I was fixing/ taking apart every machine I could get my hands on cars were the next natural progression.

Isn't it fun? I usually have a great satisfaction fixing something that someone else is stumped on! Now I'm here in a desperate mode I never thought I would be in!

 

[davesdetail]

Adjusted ours take only a little pressure to start the braking process if it's pushed some what hard it really slams to a stop. Even before adjusting the back I could hear and feel the brakes (front and back) engaging and releasing.



I don't get why the ancient AMC XJ 's with lesser brake calipers and drum size has always felt better braking.

Yes, I was told that I should upgrade to a WJ power booster and MC, but then I see that my '86 Cherokee has the best brakes out of the 3 Cherokee I have so, why can't this '89 just be nice to me and work like it should. It's not like a heavy vehicle. It's lighter than most cars I've owned.

 

[dan1977p]

I had the same issues and I will probably say my issue was one of three things but probably a combo of all. My brakes lines, mc and combo valve were gummed up from the PO's refusal to maintain them. The last three things I did after replacing everything but the actual lines and drums was replace the combo valve with the one I pulled from a WJ when I did the booster/mc swap, replacing the hard lines from mc to combo and being very, very patient when I bled the brakes using the standard RR, LR, RF, LF order.

I know the combo valve was gummed when I replaced it with a cleaned out WJ one.
I found a very tiny leak in one of the flexi lines from MC to combo when installing new combo so I built hard lines to replace.
I rushed all my previous flushes since my only helper (wife) was trying to get back inside ASAP.
Pedal doesn't go to the floor anymore and it stops much better, even though it stopped fine before, just with more pedal travel. FWIW, I was chasing this more because I wanted the same brakes (feel and stopping power) as we have on my wife's Jetta sportwagen. Neither will happen without major reworking of the entire brake system.

 

[Green Mesa XJ]

Interestingly while searching I found a story of a Corvette restoration with a soft peddle mystery, apparently they swapped the proportioning and that fixed it.

 

[techno1154]

I have been chasing brake problem with my XJ for some time but my problem was the other way, i.e. the brake pedal was very hard almost wooden feel to it and high with only marginal stopping power. I eliminated the proportioning valve (you should not do this) and now all is much better.

You should also check the placement of the lines between the master cylinder and the proportioning valve.

 

[davesdetail]

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.

 

[Ecomike]

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.

That was the method I used for 35 years until I ran into these two problem jeeps. One was fixed with new rotors that were tapered due to a set of rattle clips that were installed wrong and that wore the pads out at an angle. That was an easy fix. This wagoneer has stumped about 10 experts including me. In this case pumping the peddle out of desperation actually helped.