Your comb. valve is doing just what its supposed to do - close off one end if there is a massive leak there. Since you have air in the front lines & can't build any press., thats what it thinks is happening.
To get it back to center, sometimes stomping on the pedal like in an emergency stooping situation, it will center itself. Or, if while the pedal has press. applied, the line to the rears is cracked open, it'll shift it to center.
Once it is centered, to proceed bleeding without off-centering it again, do this. Remove the elect. switch on top & using a bolt of the same threads, make up a tool that'll screw in there & reach down to prevent the valve from shifting end to end. There's a 'low spot' on the valve spool the switch normally rides in & the ramps on the sises are what trigger the switch when it moves. Your tool should pysically prevent this motion & then it'll have to stay centered until you're done bleeding.
Doug
