Low Pedal

[rustynuts]

I am now starting to think your ABS might be releasing fluid back. That's all I can think of after all your tests. Just a thought?

[alexbrown64]

Yes, i am scratching my head on this one. I used my scanner to cycle the ABS valves with my foot on the pedal. Its works great, no issues, hard pedal goes down slightly with each ABS valve that is opened in turn. No codes etc..
I have read that a proportioning valve that is gummed up can reduce pressure to rear drums... just wondering if it can still let fluid through when you have low pressure such as bleeding.

Tomorrow i may do the ZJ mod to increase rear brake pressure, just to see what happens.

I may also need new calipers as something weird is going on.

Any other suggestions... this is going to be a pain..

[doka404]

Originally Posted by alexbrown64

Yes, i am scratching my head on this one. I used my scanner to cycle the ABS valves with my foot on the pedal. Its works great, no issues, hard pedal goes down slightly with each ABS valve that is opened in turn. No codes etc..
I have read that a proportioning valve that is gummed up can reduce pressure to rear drums... just wondering if it can still let fluid through when you have low pressure such as bleeding.

Tomorrow i may do the ZJ mod to increase rear brake pressure, just to see what happens.

I may also need new calipers as something weird is going on.

Any other suggestions... this is going to be a pain..

Gave my mechanic mate a quick call and this is what he suggests. All based on what I told him which was :

* Calipers rebuilt so assumption is that they are ok.
* No leaks anywhere in the system
* Clamping caliper lines gives firm pedal
* Only happens when engine running
* Booster checked and Ok
* No apparent power to rear brakes
* Systems bled multiple times
* No codes thrown
* Was ok when had pad change 200k ago but now, no pedal

He did say its a bit of a 'strange' problem and would really need to chuck an eye over it to see it, as its hard to diagnose over a quick phone call.

So number one - pretty sure its not the booster - but you already new that

He thinks it could be a stuck valve in the ABS unit or the master cylinder bypassing.

He said its not uncommon for Master Cylinders to go bad after doing something like rebuilding callipers as when you bleed the brakes you tend to push the MC piston further into the bore than it has ever been in its life and pick up some crud (or whatever) and do a seal. End result, the system bypasses.

So to check that, turn off the car, pump up the pedal to remove any vacuum in the system and rest your foot on the pedal with very slight pressure. If the pedal creeps and slowly sinks. Its most likely the MC.

Other thing to check is the ABS. Unplug the ABS pump (so its not working) and see how the pedal is then. If the pedal is good..then most likely the ABS system has a stuck valve or is basically cactus. He said it will probably throw a code with the ABS pump unplugged but cant say if it will be a soft code or a hard code..but if you have a scantool you should be able to clear it.

But as I said, he said its hard to diagnose over the phone and he is not a jeep specialist. He is actually a Land Rover specialist so he knows all the quirks and umm 'features' that Landrovers have. So would really need to take a look at it to diagnose fully what is the problem.

[alexbrown64]

Thanks for going to the effort to ask a mechanic mate of yours Doka. I went outside and pumped the pedal, engine off, and then apply pressure. It holds OK, and also when i start the car with the front brake lines clamped, it holds solid pressure.

I pulled all the ABS relays and fuses (4), had the ABS light on, and the pedal still drops right down. I have also used my scanner to run through the ABS diagnostics, and it tests the valves individually. I can feel each valve being switched on, as it tells me, and also the pedal drops a whisker for each valve that is cycled.

Just wondering that because master cylinders have separate front and rear circuits, if i clamp both front brake lines this effectively creates a hydraulic block and makes the pedal rock hard. Would this stop the pistons in the MC moving forward and thus making it impossible for the rear to activate. The reverse of this principle to my understanding is that if the front system blows out, the MC pistons would be able to move forward and still activate the rear brakes. Just wondering if that is why my rears don't work when i clamp both fronts.

There is some wear on the front mounting ledges where the two front brake pads sit and glide in and out. I greased them but i am just wondering if the pads can get "stuck" in the wear grooves. I have heard of people welding and smoothing this area over, so the pads glide smoothly.

Anyway, any other suggestions?

Cheers,

[Tyvokka]

Hi Alex,

I had a similar problem when:

On full left lock, I would have almost zero brakes. Pedal all the way to the bottom of the floor.

Then after straightening the wheel and pumping the foot pedal a couple of times the brakes would be okay again.

I and a few stealerships scanned my system but all was okay, nothing showed up.

Finally at Shent's, when on a hoist and doing a full left lock, they discovered that the left caliper was hitting the left lower control arm which was incorrectly welded in place. The said arm will be moved 6mm inboard to fix it.

Could it be that there could be a similar problem with your calipers, not hitting the arms like mine, but something with the front calipers?

[alexbrown64]

I am thinking there is something mysterious with the front calipers. Thats interesting with yours Tyvokka. My XJ is still unregistered but has been engineered and will be ready for inspection soon. The brakes have always been shit, but a few months ago i put in new EBC Yellowstuff pads. The difference was amazing. Solid pedal, great grip etc.. Then i put in new caliper rebuild kits and have done about 200k's backwards and forwards to the engineers. I didn't bed in the brakes as per instructions... i just kept slamming them on to make sure they were still "effective"... in that time they just seem to have gone back down hill.
I am pricing up some new crown calipers and new EBC pads to completely start again. Like i mentioned, if i clamp the front brake lines, super hard pedal with no movement.. unclamp them, brakes still grip great, but pedal goes almost to floor (within 1").

[doka404]

Originally Posted by alexbrown64

There is some wear on the front mounting ledges where the two front brake pads sit and glide in and out. I greased them but i am just wondering if the pads can get "stuck" in the wear grooves. I have heard of people welding and smoothing this area over, so the pads glide smoothly.

Funny you should mention this. Found this the thread other day when I was reading through a few forums. Does not really relate to your issue though

http://naxja.org/forum/showthread.php?t=1131547