blistovmhz
NAXJA Forum User
- Location
- Vancouver, BC
XJ master, and everyone keeps asking about the prop valve, but I don't understand how that's relevant as my problem starts LONG before the prop valve pops the rear closed.
But, just cause, I've tried both the stock XJ , as well as a ZJ 4w disc prop valve. Changing the PV had no affect because the fronts are already locked up and preventing the pedal from going further down, while the rear's can still be spun by hand.
Think of it this way. If you cap off your front output from the MC, but leave the rear connected, now hit the brake pedal. The pedal won't move because the front is already as far as it can go. If the pedal can't move far enough to push the secondary piston (for the rear), the rears aren't going to do anything.
I've also removed the prop valve entirely just to appease several other guys who were sure this was a PV issue. it's not. If I use my power seat with a block of wood wedged into the brake pedal, I push the pedal 1". At that point the fronts cannot be turned by hand and the rears can. If I push it to 2", the rears start tightening up, but can still turn by hand. The PV doesn't pop until 4".
In real world driving, 3" of pedal is enough to lock up the fronts on dry pavement. Again, tested with the PV removed entirely to the same effect.
Loads of guys run the WJ fronts with the XJ MC without issues, and loads of guys run the KJ rears with the XJ MC without issues, but I've never found anyone running both. My suspicion is that either the KJ's require way more fluid volume than the WJ's, or my MC is buggered and the secondary piston is somehow not moving far enough.
Actually, that gives me an idea. I recall losing a rear brake line last year to a tree, and I lost ALL brake pressure. I've never gutted an MC, but I'm assuming the primary piston is directly attached to the secondary, and that those two should be providing fluid isolation so that if you lose a brake line, you should still have pressure to the other end that didn't get ripped off.. Correct? If this is the case, the fact that I lost all pressure would suggest that the isolation between front and rear pistons in the MC has failed, in which case, that secondary piston is probably not really sealing well. Sound about right?
But, just cause, I've tried both the stock XJ , as well as a ZJ 4w disc prop valve. Changing the PV had no affect because the fronts are already locked up and preventing the pedal from going further down, while the rear's can still be spun by hand.
Think of it this way. If you cap off your front output from the MC, but leave the rear connected, now hit the brake pedal. The pedal won't move because the front is already as far as it can go. If the pedal can't move far enough to push the secondary piston (for the rear), the rears aren't going to do anything.
I've also removed the prop valve entirely just to appease several other guys who were sure this was a PV issue. it's not. If I use my power seat with a block of wood wedged into the brake pedal, I push the pedal 1". At that point the fronts cannot be turned by hand and the rears can. If I push it to 2", the rears start tightening up, but can still turn by hand. The PV doesn't pop until 4".
In real world driving, 3" of pedal is enough to lock up the fronts on dry pavement. Again, tested with the PV removed entirely to the same effect.
Loads of guys run the WJ fronts with the XJ MC without issues, and loads of guys run the KJ rears with the XJ MC without issues, but I've never found anyone running both. My suspicion is that either the KJ's require way more fluid volume than the WJ's, or my MC is buggered and the secondary piston is somehow not moving far enough.
Actually, that gives me an idea. I recall losing a rear brake line last year to a tree, and I lost ALL brake pressure. I've never gutted an MC, but I'm assuming the primary piston is directly attached to the secondary, and that those two should be providing fluid isolation so that if you lose a brake line, you should still have pressure to the other end that didn't get ripped off.. Correct? If this is the case, the fact that I lost all pressure would suggest that the isolation between front and rear pistons in the MC has failed, in which case, that secondary piston is probably not really sealing well. Sound about right?
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. I strongly suspect most guys don't know how the PV works, and think it acts as a bias valve during all braking operation, rather than just bias during extreme pressure.
