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loss of brake pressure

XBoBJ

XBoBJ

NAXJA Forum User
Location
Spencer MASS
hey guys i moved my jeep yesterday and no problems, i wake up this morning and goto work my pedal goes to the floor i can stop no problem but it kinda weak, my guess is a blown brake line, but when the truck is off i can pump the brakes and they get really hard and looses nothing, ive had previous brake problems and when i had a blown brake line it wouldnt have any pressure with the truck off neither, so what im getting at is there a blown line and when the trucks off it will still have pressure, or could it be something else, 99 xj stock axles with 31s 4.0 thanks..
 
just looked at it at lunch and filled the brake fluid, no visible sign of leakage, could air in the lines make you lose pressure, also im going to take the drums off after work today and see if its a wheel cylinder....
 
No brake expert here, but if you have air in your brake lines that can definitely result in soft brakes. My guess is, if you can load the brakes up with pressure when stopped, and not when driving, its probably because the brakes don't need to work that pressure off when you're parked--its just builds--which kinda gets me away from thinking its a leak because the pressure still holds. When you're driving and that brake pressure is actually used to stop the jeep, if you can't build that same pressure, you probably just need to bleed your brake lines.
 
firefight343 said:
No brake expert here, but if you have air in your brake lines that can definitely result in soft brakes. My guess is, if you can load the brakes up with pressure when stopped, and not when driving, its probably because the brakes don't need to work that pressure off when you're parked--its just builds--which kinda gets me away from thinking its a leak because the pressure still holds. When you're driving and that brake pressure is actually used to stop the jeep, if you can't build that same pressure, you probably just need to bleed your brake lines.
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i can load it up with the truck off, but with the truck on and stopped still soft
 
Ok, I didn't see you said it has drums at first. Its a good idea to see if a wheel cylinder is leaking, like you said. I still think its a adjuster issue.
 
If there's no leaks, that's a master cylinder problem. Modulate the brake pedal while stopped when it's acting up- you should be able to get it to go to the floor if it's the master. Then a quick pump of the pedal and it comes back.
 
tigerShark said:
could the vacuum line from the booster have anything to do with this?
Click to expand...
If you weren't getting vacuum to the booster it would feel like it does with the engine off. Real hard pedal.
 
JJacobs said:
If there's no leaks, that's a master cylinder problem. Modulate the brake pedal while stopped when it's acting up- you should be able to get it to go to the floor if it's the master. Then a quick pump of the pedal and it comes back.
Click to expand...
X2. A failing master cylinder will often work fine when hit hard or pumped, but sink if you push slowly down. The cups in the piston are designed to expand with pressure, so they'll still seal when you push hard, but leak down when you don't.
 
thanks a ton guys for all the imput, but i had close to nothing in the brake fluid tank, i topped it off because i tought i had a leak driving 35 miles in the city with lots of stop and go, i went down to the full mark not sure if i have a leak or a failing master cylinder because i can pump it and works alright not 100% and when i hit it hard it works but the empty tank has me stumped maybe this is out of my league and ill bring it to a mechanic, hopfully i dont get raped hasta

also dont know if this has something to do brakes but i have a couple check engine codes p0132 p0135 p0455 i know 2 are 02 senors and one is a major vaccum leak not sure if this is something to do with the master cylinder, is there any way i can check, i think the vaccum leak is my gas tank has a bad leak but i dont know if that will cause it?
 
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The hard pedal when the engine is off is normal because there's no vacuum in the booster. But the spongy feeling with the engine running is probably caused by worn brakes, or fluid that's been overheated too many times. Check your brake pads and shoes, and see if your calipers aren't seized. Also try a brake fluid flush, adding DOT4 instead to avoid this again.

There should be a flash update for your pcm that only a dealer can do to resolve the O2 sensor codes.

The P0455 means you have a large leak in your evaporative emmissions control system. That would have nothing to do with the vacuum supply to your brakes. Pull off your gas cap and look for rust stains in the middle area of the tank side. I've seen a lot of caps go bad. If that seems ok look at the vacuum line routing chart under your hood. Check all the lines and hoses for cracks, holes, or just plain being disconnected.
 
so for the o2 senor codes i dont need to change them?? gas cap was replaced jeep also runs like shit bogs down to .500rpms at idle and when clutch is in while moving also backfires out of the intake hence the reason i think its the leaky gas tank
 
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docboy209 said:
The hard pedal when the engine is off is normal because there's no vacuum in the booster. But the spongy feeling with the engine running is probably caused by worn brakes, or fluid that's been overheated too many times.
Click to expand...

this happens overnight? i moved it one night, drive it the next day as soon as i got in it the brakes were weak
 
with the jeep off pump the pedal till its firm then hold constant pressure, if the pedal slowly sinks to the floor you need a new MC
 
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