Soft Brake Pedal (after axle install)

'97 XJ: 4.0, AX15, 231, D30/9"....

i recently installed a ford 9" into the rear of my jeep using chevy K5 calipers. the guy i bought the axle off of said it wouldnt be an issue, but i had my suspicion it would be. other than the rear axle, the braking system is otherwise stock. ive honestly never messed with any brake system.

now, i plan to swap the front as well. ive read and been recommended to do the dodge 2500/3500 master cylinder upgrade (what years are we talking for this?). am i correct in the though that the larger bore size is able to move more fluid and bring the pedal right back off the top?

now, the rear is bled and leak free, i also went through and bleed it all the way around. but i feel like the fronts are largely doing most of the work... like the bias is off. if im correct in my assumption of what the proportioning valve does, is changing it or removing it something i should try before splurging on a new MC?

here is what im working with:

fronts always do most of the work, even if you have 9" drums up front and 14" carbon rotors with 6 piston calipers out back... its just physics. you can swap a ZJ prop valve, or just gut the stocker to get the rears a little more juice under heavy braking.

have you driven it yet, or just testing with the truck on jackstands? a stock 97 BB/MC should have no problem with those calipers out back and stock up front. can you still lock up the tires? if you push down the pedal to brake, lift, and push again does feel get significantly better, or stay the same?

XJLI said:

fronts always do most of the work, even if you have 9" drums up front and 14" carbon rotors with 6 piston calipers out back... its just physics. you can swap a ZJ prop valve, or just gut the stocker to get the rears a little more juice under heavy braking.

have you driven it yet, or just testing with the truck on jackstands? a stock 97 BB/MC should have no problem with those calipers out back and stock up front. can you still lock up the tires? if you push down the pedal to brake, lift, and push again does feel get significantly better, or stay the same?

Click to expand...

i agree, the front will always do most of the braking. but under heavy braking it feels like the rears just arent there.

ive put about 75 miles on it now. bleed everything twice. it will not lock up the rear. it will just barely do the front if i try to put my foot through the floor. and yes.... if i lift and reapply the brake, it does feel better.

Yes you need a master.

well, you want the front to lock first anyway.. locking the rears first is not ideal. a different or adjustable prop valve will give you more power to the rear brakes under heavy braking, but wont fix a soft pedal. unless you are putting 1 ton brakes on the front i wouldnt get the dodge master, it will be too much power and get you a stiff pedal. i've driven a few XJs with 1/2 ton brakes f/r with factory BB/MC combos and could lock up all 4 tires. you might have a problem with your booster or master... but i'm not sure. what kind of condition are you front brakes in?

fwiw, my 93 with a WJ BB/MC combo and stock 30 front, and big drums out back on a XJ44 wouldnt lock the brakes at all with 33s and a had a soft pedal... but it stopped as well as a stock XJ with ABS.

Bleed them again.

actually just drove it to class, and didnt notice the need to lift and pump to build pressure like before. i had a small leak at one of the banjos that i have since remedied.

just a thought... i have a small service loop in one of my hard lines that i did because i used a pre-flared brake line. could that potentially trap an air bubble?

Make sure the system is free of leaks and then bleed the crap out of the brakes. I run the chevy front calipers on my 14 bolt (3" pistons) and the 3" t bird calipers up front on my 44 and can stop no problem with a stock WJ master, which I think is the same bore as the XJ? anyway, make sure all lines are tight. are you getting more air at one of the bleeder valves still? that will help you locate where you might have a leak. sounds like air. did you let the whole system go dry when you swapped the rear? If so, you need to bleed the MC and the prop valve too.

I'm about 3 minutes from starting my PM class so I'll try and keep it brief. A few notes, though
A "soft" brake pedal is a tricky way to describe it. If the pedal goes to the floor / sinks too low and braking is weak, you probably have air in the line or a leak. If the pedal goes 1/4 or 1/2 way down and then feels firm and the brakes work (after a brake upgrade I mean) then you aren't moving enough initial volume to get a good brake feel - but it's not a problem, per se.
If, under any circumstance, you have to pump the pedal to make it stop : that is a problem in the brake hydraulic, either air or leakage or blockage but it's not OK and not normal, even when you add larger brake calipers.
A loop in the hard line can definitely lead to a trapped air bubble. Same principles as plumbing coolant apply.
...aaaand, lecture is starting, so no time to clarify / elaborate. Hope this helps.

i have bled and bled and bled... zero leaks. relatively confident that there is no air in the system, i pinched the line off to avoid a mess and dont believe i let the master run dry, but i am also not confident either. there is no need for pumping or dead spot off the top. the pedal is there, but farther down the stroke. at this point, i can only think of bench bleeding the master and proportioning valve just to be double sure.

with that said, im thinking of just replacing with the dodge 2500 master, as ill be replacing the front axle in the near future as well. any issues seen with this?

VAhasnoWAVES said:

i have bled and bled and bled... zero leaks. relatively confident that there is no air in the system, i pinched the line off to avoid a mess and dont believe i let the master run dry, but i am also not confident either. there is no need for pumping or dead spot off the top. the pedal is there, but farther down the stroke. at this point, i can only think of bench bleeding the master and proportioning valve just to be double sure.

with that said, im thinking of just replacing with the dodge 2500 master, as ill be replacing the front axle in the near future as well. any issues seen with this?

Click to expand...

ok was the master dry or not when you popped the cap and went to bleed it after the caliper install ?

Also, thanks for explaining now. I have the same thing, the longer pedal travel, but it stops on a dime. its weird.
if thats the case, and you are 100% sure the MC and prop valve are bled, then its just the MC just not moving enough fluid, which at that case you want to make small increases in size, go up 1/16" bore at a time.

since you have to pull the MC to bench bleed, go ahead with the 2500 MC and post pics and part numbers and comments on fitment

Make sure you don't have a warped rotor. It can cause a spongy pedal.

What size are the bore in the calipers? They look quite larger and you may not have enough master cylinder for them.