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Spongy pedal-firm pedal?

bcmaxx

NAXJA Forum User
Been fighting with a funny brake pedal since I have owned my jeep-8 months now (89 xj,4.0,auto,no abs) It has an intermmitent spongy brake pedal. At first I replaced the master cylinder,pads,shims and calipers,checked booster vacuum and replaced hose to booster,no luck pedal seems to go darn near to the floor but it still stops,I drove my friends 88 xj and I couldn't believe how good and firm the pedal felt,night and day difference,so I slowly replaced the following:
front brake lines and hoses,rear flex hose,rear drums,shoes,cylinders and steel lines,brake adjusters and adjuster cables and guides,calipers again,bled,bled,installed speedi bleeders,bled and bled,no change in pedal until recently. Every once and a while I'll hop in my jeep go for a drive and hit the pedal and its firm,brakes very well just like I think it should and three hours later back to the sponge. Its firm sometimes and back to crap, temperature ambient and engine doesn't make a diff,neither does rpm,wet damp or dry?Anyone have a clue whats going on? I manage an auto repair shop so I'm not a backyarder,none of my techs really have a clue either.
 
You've covered it all, but... I had crappy brakes on my 90 Comanche, until I replaced the booster. I checked vacuum, replaced hose etc. Supposedly the test for a booster is-put foot on pedal, when you start engine the pedal will move down a little more, and mine did it. Nagged at me, Then when it came time to turn it over to my 16 year old, I knew I had to do something. I just broke down and bought a rebuilt booster. It was only about $70 from O'reilly's. Pain in the butt to replace, I might add, thats the main reason I didn't want to do it. Fixed! Good luck!
 
Booster failure might be one suspect if the intermittent change is accompanied by a change in pedal effort, but if the pedal is going low without a change in boost, it's unlikely.

How about the proportioning valve? That can cause rotten braking, although I don't think it usually affects pedal height.

If the effort remains the same but the pedal sinks lower sometimes, the master cylinder is one suspect. You could have gotten a bad one, and that's one thing to check. I've had pretty poor luck in the past with rebuilt master cylinders. Try leaning steadily on the pedal with medium effort, and see if it sinks at all.

Check your wheel bearings, too. A wobbling wheel bearing can cause intermittent soft brakes.
 
i once had a guy bring me his jeep with soft brakes and he had replaced the front calipers cause one was leaking from the piston. he bled and bled and bled the brakes but couldnt beat this soft pedal. i had a look and he had mixed up the right and left caliper and the bleed screw was of course backwards and all the air couldnt get out. flipped em around and it was ok. maybe your calipers are back asswards, just tossin this out sounds like you tried everything so the fix may seem far fetched at this point. good luck
 
Yeah,I bench bled the master, and I have ensured the calipers are on the proper side,I think I'll try a wrecker brake booster to begin with. After that I'm down to lines and proportioning valve. Bad news is I'm near broke after the sensor replacement game for rough idle/lackof performance,winch/bumper/lift/tires.Good news is It wont have much else to go wrong (knock on wood)
Thanks for the suggestions.
 
Of course badly misadjusted rear brakes would also cause a low pedal, but I don't see any simple explanation for it being intermittent. Still, it's probably not a bad idea to open them up while you're at it, and make sure there isn't anything funny going on there.
 
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