Or rather, combination valve, since that's what the factory service manuals insist on calling it.

What exactly does it do and how exactly does it do it?

I understand that it limits the fluid pressure going to the rear brakes. Mine seems to be functioning backwards (lots of pressure to the rears, not much to the fronts), so I want to know what's going on.

What is the disadvantage of replacing the factory combination valve with a straight pipe for the front brakes (actually, I think you'd need a junction block, but you get the idea) and using an adjustable proportioning valve on the rear line?

 

the factory combo valve is called that because it's actually more than just proportioning ... ALL early cars had to have a valve to recover from a fluid loss in the system .. commonly called a DOT valve ... i.e. if you loose a seal in a front caliper , the system blocks off the front system and keeps the rear system intact ..Volkswagen splits the system diagonally .... GM and most US makers split it front and rear ... this is done with spool valves in the combo valve ... usually 2 triple stack valves , attached together .... if you know your hydraulics , there's a proper name I'm forgetting here ... single pull ??? ... anywho ... usually the failure is corrosion that locks the valve assembly .... so you loose one system or the other ..in one fashion or another ... another common symptom is pedal depth ... you've replaced the entire system and the pedal still goes to the floor ..even with the rears full adjusted ... I've seen this on several GM trucks ... or in your case ..I think ... it's closed the fronts off and you have rears only ?????? Generally if you mess with the valve ( taking it apart) it's junk !!! .. I've heard they are under $50 from GM ... let it gravity bleed before pressure bleeding !!!

TVP