Brake problem question

[RichardJ]

The "Combination Valve" includes the "Metering Valve", "Pressure Differential Switch" and the "Proportioning Valve". Three valves in one. The "Metering Valve" is sometimes called a "Hold-Off Valve".

>>The prop valve has a hold-off valve in it that engages the rear brakes first for light braking to prevent nose dive, so maybe when the hold-off is surpassed and the fronts engage they grab.<<

It is only semantics, but to put it a different way, the "Hold-off Valve" prevents the front brakes from working until pressure reaches a certain point. The pressure point varies by vehicle, but let's say 50-100 psi.
The rear drum brakes are unaffected by the hold-off valve. The rear brakes would act normally, not knowing if the hold-off was bad or even if there was one present. The proportioning valve doesn't play a role until pressure is upwards of 500 psi. The proportioning valve effects rear brakes only, and only above 500 psi.

MC is leaking - bad MC

Differential switch is ON - number one suspect at this point is bad MC.

Bad nose dive sounds like hold-off valve is working and you have little or no rear brakes - bad MC.

I would caution anyone from taking a combination valve apart in hopes of repairing it. If you damage any seals or valves, there are no replacement parts or kits.