There have been a couple of things discussed here that are, in a single word, just plain WRONG.
I'll start with the pin sticking out the front of the combination valve. That is the METERING VALVE and has absolutely noting to do with the "Pressure Differential Switch". That is the name for the light switch you've let yourself get side tracked with.
The COMBINATION VALVE is just that. A combination of three separate, unrelated devises combined into ONE package.
Some factory COMBO valves include a residual for a fourth.
There is no reason you can't use a proportion valve in the combo valve, PLUS a second proportion valve. It would have one distinct advantage. You would probably never, ever wear out the rear brakes, because they would only be lightly dragging at best. The front disc will do 95% of the braking and will wear much faster.
Most Pressure Differential Switch's DO NOT cut fluid flow to the side the has a failure. It simply turns the light on. I would hope that if their combination switch did have that feature of stopping the flow, they would have made sure you were well aware of it. It can make it harder to bleed and wouldn't be what they would want to sell to DIY'ers.
The metering valve that is in the combo valve is a good thing. It keeps your girl friends head away from the dash or sun visor when you first jump on the brakes. Some cars are more susceptible to nose dive than others.
Some metering valves will require you to EITHER hold the pin IN or pull it OUT in order to bleed the front brakes. Ask the people that supplied the kit.
What ever you do, logic should tell you, that you need to fix the leak FIRST. Logic should also tell you that after looking at several other calipers, the problem is the bolt, the hoses or you.
Try this web site. It does a great job of explaining the various brake valves.
Combination Valves
