Thank you Ranz.
Embarrassingly this looks my own fault!
I went to check my one way return valve and there isn't one.
My car has a twin spark and a couple of years ago I converted to throttle bodies and aftermarket EFI, so I got rid of the standard 75 Plenum, I left the one way return valve attached to that!
And with my new AHM manifold I tee's straight in with no check valve!
I'm a donut! I've ordered an inline check valve now!
Hopefully that sorts it, might also apply a mild vacuum to the diaphragm using my mityvac to see if it holds a vacuum and is all good!
That funny brake feel thanks to me omitting a £5 part!
1966 Giulia Sprint GT Veloce- Twin Spark Converted