Originally Posted by larock
Where and what is the combination valve?
it is this block that your brake lines pass through. It is a "combination" valve as it performs several functions.
In this diagram from the FSM you can see how a special tool is used to keep the metering valve open during bleeding. Sometimes they bleed and sometimes this valve closes, believing you have lost pressure in the rear. You can keep it open with a screwdriver or something, no need for the tool.
If in any doubt you can get a copy of the FSM on the 'Net and read up on the installation, adjustment and bleeding of the brakes.