brake issue need advice

[98CherokeeSport]

i have a 1998 cherokee with stock braking system and no ABS. so my brakes were sticking the other day so i thought my caliper piston had died again. so i replaced both front calipers and then bled all the brake lines. i pumped up the brakes and got good pressure when the jeep was turned off. i then started it to take it for a spin and as soon as it started the pedal went straight to the floor and stayed there. i couldnt get any pressure built up. so i then replaced the master cylinder. i bench bled it before i installed it then after i installed it i bled all the lines two more times. i still had the same results. pressure when the jeep is off and none when it is on.

could it be the brake booster? or what else could it be? i cant find any information on this exact problem

 

[winterbeater]

It shouldn't matter, but I'd try bleeding them with the engine running. Not sure why, just an idea.

 

[98CherokeeSport]

i thought of that too. so i bled it with it turned to accessory to get everything working. still didnt work

 

[kastein]

I always bleed my brakes with the engine running, it's never failed me. Having the booster working makes it a lot easier and I am lazy.

 

[98CherokeeSport]

i guess i will try it while its running instead of on accessory and see if that helps

 

[98CherokeeSport]

so i started the jeep and bled the brakes and it didnt fix anything. any clues?

 

[winterbeater]

OK, so pedal goes to floor. Does it pump up to where you get pedal, or always go to the floor even after repeated pumping. (with engine running)

 

[98CherokeeSport]

when the engine is turned off i get a lot of presure. but when it is running it goes to the floor and no matter how much i pump the pedal i still dont get any pressure at all

 

[joe_peters]

Pascal's law (päskälz') [for Blaise Pascal], states that pressure applied to a confined fluid at any point is transmitted undiminished throughout the fluid in all directions and acts upon every part of the confining vessel at right angles to its interior surfaces and equally upon equal areas. Practical applications of the law are seen in hydraulic machines.


Report to Area 51--you are in possession of restricted alien technology that defies physics.


Seriously, pull things apart and examine the individual components. The only way the pedal can go to the floor is if there is air in the system or something mechanical between your foot and the hydraulics is busted. As you have pedal engine off and no pedal engine running I would would start with the pedal, the pedal mounting bracket, the rod to the power booster, the power booster, and if after pulling each of those items from the vehicle, and giving each part a physical examination under good lighting while holding it in your hands, then you post back here and let us know what you have found.

 

[Banditt007]

The ONLY significance of having the engine run while messing with your non ABS brakes would be b/c it is causing the booster to 'work' b/c of the vacuum. If you were to remove the vacuum line from the booster and run the engine, i bet the brakes would react just the same if the truck was off.

How did this problem start, completely out of the blue? Did you do brake related work recently?

 

[JJacobs]

Did you put the calipers on the wrong sides? Bleeder screw goes to the top.

 

[89eliminator]

im having the exact same problem. how does one check the booster for leaks?

 

[lawsoncl]

It's not the booster if its building pressure with the engine off, but going to the floor with the engine running. That means the booster is working! You have air in the system or a bad master. Make sure you have the calipers on the right side too.

 

[89eliminator]

for me, i dont think its air in the system because if that was the case, usually if you pump the brakes they build pressure. mine do not. i just put in a new MC too.

 

[ParadiseXJ]

Make sure you have the calipers on the right side too.

X2, I did that very thing and got the same symptom, just wouldn't bleed or build any pressure. Older and wiser now.

 

[Reclipse]

Here is a thought, did you start bleeding the brakes furthest away from the MC?

 

[89eliminator]

Here is a thought, did you start bleeding the brakes furthest away from the MC?

yes, did that.

 

[lawsoncl]

for me, i dont think its air in the system because if that was the case, usually if you pump the brakes they build pressure. mine do not. i just put in a new MC too.

I've had cases where way too much air in the system results in the pedal not building any pressure.

 

[89eliminator]

i've literally bled my brakes 10 times now. i also cracked the bleeders and just let them gravity bleed a few times. i gotta believe there is no air.

 

[joe_peters]

Sell it or take it to a brake shop.