yj brake pedal goes soft @ low rpm

[hamrtime]

89' yj

been running great long time

brake pedal firm at startup and driving but
when i pull into parking lot and putt around
low rpm then try to park pedal gets really
soft.

first occured week or so after getting new
shoes installed on rear D44 [metric ton cherokee],
i bleed the brakes couple times on weekend, put
in new master cyl but remains the same.

is there anyway to install D44 shoes/spreader bar
parts backwards  or does this sound like booster
issue or vac to booster  ?

appreciate the help, thanks

[TL-Iguana]

disconnect the vacuum hose going into the booster and clamp it off with a vise grip. Then try it. It would rule out the booster which might be a factor. Low RPM would lead to low vacuum.

Not a mechanic, but this seems to stick in my head as what the fix was to drive a vehicle to the shop with a blown booster.

HPS4EVR might have some more insight if you ask him, otherwise id suggest you take it to Broken Axle and talk to those gents in person.

[cLAY]

If it was a blown booster the pedal would be hard and the engine would run like crap as if it had a massive vacuum leak.

I think you are on the right track with the spreader bar/adjuster star wheel. They can be installed backwards and they are different threads from each side, one side is regular thread the other is revererse(left) thread.

What sounds like is happening is your brakes are backing off, the first pedal application takes up the slack and the second then actually applies the brakes. This is often seen with disc brakes that have badly warped rotors that push the piston back into the caliper as they turn.

Did you properly adjust the rear brakes after installing the shoes? You have to manually adjust ups the star wheel to take up the slack after servicing, once its close the mechanism will keep it adjusted after that., provided you don't get it full of mud. 

A severely warped drum or axleshaft could also be pushing the shoe back, not as likely though.

[hamrtime]

thanks for advice, got some things too check now

the warped front rotors scenario something I didn't
consider so if all else checks out will take them in for
machining/replacing

[Dingleberry]

What places can you take your rotors in to get turned?

[hamrtime]

thought pretty much any brake shop can turn [lathe] them

pair of new yj rotors is < $100 which I would just do
but I have warn hubs so center hole has to be machined
making them +$200 value to replace new