1987 Carrera 3.2 Cabriolet brake problems

[pbmitchell]

I have a 1987 Carrera Cabriolet that I bought in 1988. It's done only 50,000 miles from new but it hasn't been on the road in the last 10 years. It lives in my garage and comes out occasionally for a run up and down the driveway.
Last year or thereabouts, I realised the calipers had seized and I replaced them all.
After replacing the calipers and bleeding the brake system, all seemed well until I realised something was amiss.
With the engine off, the brake pedal feels normal; I can depress it a short distance and then it feels solid.
With the engine running, the servo (booster) operates and again the brakes feel normal. I can depress the brake pedal a short distance and then it firms up and the brakes work - BUT if I continue to press, I can press the pedal all the way down to the floor and, while it's travelling this 'excess distance', it feels as though I'm pressing against a spring of some sort
Clearly, something isn't right so any ideas or suggestions would be very much appreciated.

[Flat6pac]

Sounds like you may be bleeding by the O rings inside the master cylinder.
Bruce

[pbmitchell]

Thanks, Bruce. I assume this means I'll have to replace the master cylinder?

[OffCamber00]

Yep, sounds like a bad MC. Not surprising given the lack of mileage. Seals dry out and then you get the symptoms you are describing.

[pbmitchell]

Thanks, Mat P. I see what you mean.