Jim Malcolm
NAXJA Forum User
- Location
- PA
Just so everyone is clear, there is no actual proportioning valve in a XJ. Google search "xj cherokee proportioning valve site:comancheclub.com", go to the Images tab and you'll find one cut open. The other is from an MJ with a cool little rear bias "proportioning" valve based on how loaded down the rear end was. You will not find a proportioning valve. Other than the shuttle valve for the warning light, there is zero connectivity between the front and rear circuits. By all indications, it's a metering valve, setting a minimum pressure that must be achieved before allowing the fluid to pass. By all means, prove me wrong, I'd like to know as much as anyone else. It looks more like a residual pressure valve, but the fact that the only difference with the ZJ is a stiffer spring leads me to believe otherwise. More residual pressure is not what you want with discs.
You'll see that the spring cavity will trap air; I'd bet money this is where your problem is. There's a bleeder built into the cap; you can bleed the rears from here to eternity and you won't get the air out otherwise. The front is generating full pressure, keeping the MC from stroking any further and it's compressing air in the rear instead of applying pressure at the pads. This is the safety mechanism against rear brake failure in action. In the case of front brake failure, the shuttle valve shifts to the left, opening the passage that bypasses the valve and the rear does the best it can...
Unless you have different front calipers, the LAST THING you want to do is increase the MC bore diameter without also swapping in the matching brake booster. If you want to increase performance with stock front calipers you need to decrease MC bore diameter, sacrificing pedal stroke and creating "softness" to the point that they get soft-soft-soft-lock em up because you're generating really high pressure but only once you've stroked it a good ways to engage the pads. If you've done the WJ brake swap or have tons, you need to do the math.
You'll see that the spring cavity will trap air; I'd bet money this is where your problem is. There's a bleeder built into the cap; you can bleed the rears from here to eternity and you won't get the air out otherwise. The front is generating full pressure, keeping the MC from stroking any further and it's compressing air in the rear instead of applying pressure at the pads. This is the safety mechanism against rear brake failure in action. In the case of front brake failure, the shuttle valve shifts to the left, opening the passage that bypasses the valve and the rear does the best it can...
Unless you have different front calipers, the LAST THING you want to do is increase the MC bore diameter without also swapping in the matching brake booster. If you want to increase performance with stock front calipers you need to decrease MC bore diameter, sacrificing pedal stroke and creating "softness" to the point that they get soft-soft-soft-lock em up because you're generating really high pressure but only once you've stroked it a good ways to engage the pads. If you've done the WJ brake swap or have tons, you need to do the math.