Hi . I have just had a similar problem on my 1974 plus 8. I drove the car out of my garage and on braking my foot went straight to the floor so I put in a new master cylinder. We then went away in it for a few days, returned put it in the garage. When I next went to drive it the same thing happened again my foot went straight to the floor.
I bypast the servo and the brakes have worked fine since. At present the servo is waiting to be sent to Terry Foxton for rebuild but the brakes don't feel any different than they did,so I'm not in any hurry.
Just my expereance in case out helps.
Steve.
