Low Pedal

[alexbrown64]

Pedal on XJ Cherokee goes low only when engine starts.
Booster checks out fine. Pedal is hard and high when engine is stopped. Turn engine on, and pedal drops (almost to floor).
I bled entire system 4 times yesterday, including using scan tool to cycle ABS in bleed mode. Today i bled 2 more times including pulling calipers off (leaving line attached), and pushing pad all the way out and back in while bleeding nipple into bottle.
Rotors are in great condition, run kits through calipers at xmas, nothing leaking, no air bubbles, new rear wheel cylinders at xmas, nothing leaking, adjusted rears right up today to remove any slack. New EBC Yellow stuff pads at xmas which really pull the Jeep up. Brakes still work well, but pedal goes really low when hitting the brakes hard.
Measurements are: Pedal 125mm from floor stock. then almost hitting floor with full pressure.

I eliminated master cylinder by clamping the front brake hoses. Pressure holds and pedal only goes down 25mm too 100mm from the floor,with both hoses clamped. If i clamp either left or right hose (anywhere on hose, top or bottom), pedal goes down to 75mm from floor. If i take off clamp, pedal drops down to within 30mm of floor. So to reiterate, if i clamp front hoses near calipers, then the pedal is stiff and only drops marginally. Its like the calipers are taking all the pedal travel. When i removed the calipers, i greased the pins and made sure they slide nicely when mounted. I also bled the crap out of them using a helper method and a bottle method.

Any suggestions as to where my pedal travel is going? Calipers are originals with new kits in and don't appear to be leaking. I even took them off and inspected all around them.

When i put the EBC pads on, brakes felt strong and great with not much pedal drop. Now, after only 200k's of driving, the brakes seem to be going bad again.

I don't believe it can be booster, master cylinder or rear brakes, ABS or any part of the system up from the calipers, as everything works great when the hoses are clamped just up from calipers.

Cheers,

[alexbrown64]

OK, i just ran an experiment. I went to Supercrap and purchased two mini hose g clamps. I put them on the front XJ brake hoses near the calipers and went for drive. The brake pedal was stiff and did not drop down. Took them off and the brakes came on but pedal went almost to floor. Something about these calipers is causing a huge pedal drop?
On another note, when front hoses were clamped the rear brakes would hardly work. I always suspected that the XJ rear brakes hardly did anything, but if i really press hard on the pedal it will hardly pull up. I would say it just stops it rolling away from traffic lights when in Drive and that's about it. Proportioning valve? The rears bleed perfectly, no blockages and yesterday, i had the back jacked up and in Drive and the brakes seem to work fine in that position. Any ideas?

Cheers,

[rustynuts]

How are your front brake hoses? They may look fine but old hoses can swell under pressure resulting in the excessive pedal travel.
Try having a mate push the pedal with brake boost while you hold your hand around the hoses. My brake hoses were good but I got an even firmer pedal when I went for new braided lines.

[rang0]

So your saying that its got pressure when the car is off but when you start it theres no brakes? Try disconnecting the vacume line to the boost to see if theres any difference. There shouldn't be any difference. It could be a swelling brake line or you booster might have something going on. The swelling brake lines make sense. When you clamped the line was it on the top or the bottom of the hose?

Sent from my SM-J100Y using Aussie Jeep Offroad mobile app

[doka404]

I would think that if the soft lines are expanding you would get a spongy pedal rather than it going to the floor?? But like Rusty when I went to braided lines the pedal was noticeably firmer. Maybe there is a pin hole leak that is letting air in but not necessary giving any signs of a leak? Maybe at the fitting on the caliper?

With the car turned off and a firm pedal, if apply constant pressure to the pedal does if stay where it is or move slowly go down? If it slowly sinks to the floor then sounds like the MC has a problem.

Do you get any bad idle or such like? If you do this could indicate a bad booster (Vacuum leak) may also hear a hissing noise in the booster. I have only seen a bad booster cause no assist (ie hard pedal but no real pressure) rather than pedal going to the floor.

Ok now for a really obvious question! The bleed nipples are at the top of the caliper aren't they? I only ask as I had a mate who had a similar problem and asked me to come and help. Looked at his car and yup..calipers were on the wrong sides an upside down I think i alreadyy know your answer to this one thoguh

From my experience (and I am not saying it the only cause) with excessive pedal travel but still good brakes (like you mentioned), its either something like pads getting knocked back from a warped disk or bad unit bearing etc. or its air in the system (assuming the MC is good).

You could always remove the lines from the MC, block them off and see how the pedal is..if its good then MC should be ok

just my 2c

[alexbrown64]

I have tried clamping at both ends of the rubber brake line so that i included in one test and isolated it in another. It made no difference to the brake pedal drop measurements.

I do have brakes. The front almost locks up when i apply full pressure to the brake pedal. Booster working fine and passes all the tests in FSM and Haynes.
Its just when you start the car and have your foot on the brake pedal, it drops right down near the floor. If i clamp one front brake hose, its not as bad, and if i clamp both front brake hoses the pedal doesn't drop at all.

However, when i clamp both front hoses and take a test drive, the rear brakes hardly work. The rears flow and bleed fluid freely, are adjusted correctly and no leaks. With the front lines clamped just before the calipers, the brake pedal is solid and does not depress at all. Its like the rears are blocked. Thats why im wondering if the proportioning valve is malfunctioning.

Cheers,

[doka404]

Originally Posted by alexbrown64

I have tried clamping at both ends of the rubber brake line so that i included in one test and isolated it in another. It made no difference to the brake pedal drop measurements.

I do have brakes. The front almost locks up when i apply full pressure to the brake pedal. Booster working fine and passes all the tests in FSM and Haynes.
Its just when you start the car and have your foot on the brake pedal, it drops right down near the floor. If i clamp one front brake hose, its not as bad, and if i clamp both front brake hoses the pedal doesn't drop at all.

However, when i clamp both front hoses and take a test drive, the rear brakes hardly work. The rears flow and bleed fluid freely, are adjusted correctly and no leaks. With the front lines clamped just before the calipers, the brake pedal is solid and does not depress at all. Its like the rears are blocked. Thats why im wondering if the proportioning valve is malfunctioning.

Cheers,

I would think that if the proportioning valve isn't working (ie stuck biased towards the front) you would not be getting much fluid out of the rear wheel cylinders?