MJ Proportioning Valve - Answers!


New member
I think I've figured out the MJ setup!

The front prop valve has two lines going to the rear. One line goes to the height sensing proportional valve above the rear axle and continues to the rear brakes. That's the one that normally gets used. The second line goes to a Tee in the line after the rear prop valve. That one gets used if something goes wrong front brakes and bypasses the rear prop valve to give the rear brakes full, un-proportioned pressure.

The prop valve on the MJ doesn't seem to do any proportioning. It just has the shuttle inside that kicks back and forth if one circuit has significantly less pressure than the other and trips the brake light. If the front loses pressure, the shuttle slides and opens up the second line to the rear brakes - the one that bypasses the rear prop valve.

All of this made me realize one very important thing (well that and a tip from a Google search). The second line doesn't get bled when you do a normal bleeding! I found this procedure to follow after you do a normal bleed.

1. Open a front bleeder and pump the pedal until the brake light comes on. This will cause the front combo valve to let fluid go to the second rear line.
2. With the front bleeder still open, bleed the rear brakes again. If vacumn or gravity bleeding, you can close the front bleeder as long as the brake light stays on.
3. Close the rear bleeder and re-bleed the front lines.

You might be able to crack open the line on the nose of the combo valve with someone pushing the pedal to bleed the second line as well. Have not tried that yet.

Second thing I realized is that the best way to eliminate the rear valve and get full pressure to the rear, is it to replace it with a coupling. You still need to bled as above. I still have mine in place.

After doing the procedure above and aiming the rear prop valve lever straight up, I can lock the rear brakes!