Here is the "by the book" procedure:
Test/Reset Combination/Proportioning Valve:
Use a test light by attaching a clip to a positive contact on the vehicle and touch the point of the tester to the electrical connection of the combination valve. If the the light does not come on, the valve system is operating correctly and no further testing is required.
If the light does come on, this indicates that the pressure differential valve is stuck in the front or rear position.
Bleed the brake system to determine if the front or rear lines are blocked off. Set up one front wheel and one rear wheel for bleeding at the same time. Crack both bleeder screws and gently pump the pedal a few times.
The blocked side will trickle fluid out when the bleeder screw is cracked and the pedal pressed. An unblocked line will squirt fluid out the bleeder.
The lines that are clear must be left open and the blocked lines should have the bleeder screws tight to cause pressure to build up on that side. Be sure to use the standard bleeding procedures to prevent air from entering the system.
Slowly press the pedal with steady pressure a number of times until the light goes out; this will center the differential valve.
You may also hear a pop come from the proportioning valve. This is the metering valve returning to its equalized position. When the light goes out, close the bleeder screw. (See fig. below)
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Having said all that - I can usually get the piston to recenter (and put out the light) by just slamming on the brakes a few times. Go forward and slam, then go in reverse and slam. It usually shifts the piston.
Make sure your parking brake isn't engaged - done that a few times.
