Anyone who tells you that you do not need a factory combination valve in a disc/drum setup has no clue how a braking system works and does NOT have a braking system that stops just fine.

Get a book on braking system or find an FSM that will explain to you what each part of a combination valve does and Danan you are going to get someone killed with your ignorance.

A factory combination valve, what most people mistakenly refer to as a proportion valve performs 4 functions.

1, pressure differential, this is what Danan was confused about. All pressure differential does is light up an idiot light to tell you that you lost pressure in one side of the brake system. In the old single reservoir setup no brake pedal pressure told you that, well no brakes too.

2. hold off, this delays application of the discs until the shoes are touching the drums

3. metering, this decreases the rate or pressure increase to the rear drums. Modern drum brakes are self energizing, this means that as they actuate they tend to dig in harder on their own and if you do not back off on the pressure increase they will lock up.

4. proportioning, this limits the flow and pressure applied in a panic stop. In normal operation it rarely comes into play. Aftermarket adjustable proportioning valves are not true proportioning valves but rather a pressure regulator that limits maximum pressure. True proportioning valves use the disc pressure as a reference to control the rear pressure, something an aftermarket adjustable valve cannot do as it is only plumbed into the real line and has no reference.

So the next time someone tells you that you do not need a combination valve and that an aftermarket adjustable valve "works fine" you will understand their full lack of their knowledge.

Just remember this is only applicable to a disc drum setup, not an all disc or an all drum setup.