Front Brake problem....Maybe

[Jeep839]

So i just replaced the front calipers, brake pads, and rotors on my 90 XJ in front and then adjusted the rear drums after this purged the lines with a vacum pump to get ride of any unwanted air and then i was on my way. So the Question i drive and while i drive my brake pedal is hard and up nice and high where it should be with new pads, but then i go in my house let my truck sit for like 30 mins and my brake pedal losses pressure it goes all the way to the floor and then when i pump it up a couple of times it is back to where it is suppose to be, any cure for this or what might be the problem???

90 XJ 4.0 aw4 np242 D30 D35 laredo

[JVRacer04]

did u get ur pedal back or do u still not have any pressure?
id bleed the brakes again or check ur proportional valve


I just swapped rotors calipers and pads too... wow i thought that was a huge difference till i did a rear brake job yesterday
that made all the difference and my brake pedal still had alot of play until i did the rear brakes, now i can literally stop on a dime

[Jeep839]

i get the pressure back when i drive for a couple of minutes and then if i sit for like 30 min the pressure is gone. the rear brakes are fine replaced them not to long ago and as for the proportioning valve why would it give me a problem now i mean i checked it and it's not leaking or anything?

[mudpuddle]

So i just replaced the front calipers, brake pads, and rotors on my 90 XJ in front and then adjusted the rear drums after this purged the lines with a vacum pump to get ride of any unwanted air and then i was on my way. So the Question i drive and while i drive my brake pedal is hard and up nice and high where it should be with new pads, but then i go in my house let my truck sit for like 30 mins and my brake pedal losses pressure it goes all the way to the floor and then when i pump it up a couple of times it is back to where it is suppose to be, any cure for this or what might be the problem???

90 XJ 4.0 aw4 np242 D30 D35 laredo

Sounds as if your master cylinder is leaking. Sometimes it happens by leaking by the seals internally, you won't see any fluid leaking. When you "pump it up" it brings the piston back to normal position by pulling in fluid to the proper side of the pistol and cylinder. You can check this by holding steady and even pressure on the brake pedal and see if it "leaks down" towards the floor over 30 seconds or so.

[Bradlybob]

When you try the brakes after letting it rest for a while are you starting the engine or just pushing the brake pedal? I kinda sounds like a vacuum leak in the master cylinder.

[Jeep839]

why would it only give me trouble now. when i changed the calipers compared to when yesterday i had the old old brakes and calipers on and it didn't give me that problem?

[CheapXJ]

you don't have the calipers backwards do you?

bleeder screws should be on top.

otherwise, keep bleeding until you have completely clean fluid coming out.

it's possible that moisture was in the system, and rusted the bore of the master cylinder.

[Jeep839]

calipers are in the right orientation and i am, driving it like usual and then the brake pedal gradually comes back up

[Matthew Currie]

When you try the brakes after letting it rest for a while are you starting the engine or just pushing the brake pedal? I kinda sounds like a vacuum leak in the master cylinder.

A working vacuum booster will cause the pedal to drop when you start, and if it is leaking down when it sits, the pedal will be higher rather than lower.