Chris,
The Brittain is really a simple system ot troubleshoot. If you don't have one buy an Mooney maintenace manual off E bay. It need not be current as vacuum still work the same way! The manual has a great section on the PC system and Brittian is willing to send additional manuals by email. Follow in the procedures in the manual and you should be able to locate and repair your troubles.
I have a B6 in my 66 E model and flew 3:45 at 11,000ft in heading mode with no nav input other than twisting the heading bug, but it flies just as well in PC only. Vacuum servos are cheap to overhaul and parts are always on E bay.
Clarence,
M20E C-FMYB
M18 C-GMYT