Marlin, be careful of the used master cylinder's. There are seals in the master that go bad and allow fluid to slowly flow past them which is why the VX will stop initially (seal isn't bad enough to let the fluid just blow by it) and when additional pressure is applied the pedal goes to the floor (fluid gets slowly forced past bad seals). I have no idea where these mythical seals are, and if they can just be replaced, but a master cylinder that has been sitting around with little to no fluid in it may have busted seals already and could REALLY put you and your VX in danger when you try to stop.

I was told by Merlin that the booster rarely goes out, and that it's usually the master. I just replaced my master cylinder and left the old booster in place and so far all is well.