[Mike K]

I have a few spare dissys , all used . I fitted a NOS dissy on the C8 , it was in a box of spares.

Maybe the rubber diaphram has perished in your vacuum unit ? It is possible to pry the unit apart and replace the rubber on some types. It's a fiddly job , but might be possible on the GM unit ? . I did this to my series 1 Land Rover vacuum unit and it was a successful fix.

I have one of the hand pump vacuum tester units , these are very handy and not that expensive on EPAY. You connect it to the vacuum pipe and watch for the dissy to rotate as you pump.