My MP gage behaved identically. I could get the gage to go full deflection when I keyed the mike. Then it would go full deflection when I turned on the master switch. Cycle the master and the gage would stay at zero, and never work at all again.
I pulled the whole Vans MP system from the plane and replaced it with an old fashioned mechanical/pneumatic gage and copper tube plumbed directly into #3 cylinder head.
Five Sierra Fox