Yeah, I have read that the "plunger" is suppose to be clamped with a big C-clamp and a piece of wood to stop the proportioning valve from blocking out either the front or rear brakes during the bleeding process. I don't have a "plunger" on (what I think is) the original equipment proportioning valve.

When I bleed my brakes I use one of those "one man" vacuum bleeder kits from HARBOR FREIGHT. They're cheap but it works pretty good for me and I don't have to worry about the proportioning valve blocking out the front or rear brakes as during "traditional" bleeding by having someone pumping and holding the brakes while someone else cracks open the bleeder at the brake.

Any way here's a good way to "reset" your proportioning valve:

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; 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. Then close the bleeder.

Hope this helps. If it doesn't clear the problem then you might have a blocked brake line.