A very strange brake issue

i have a military series 3, the brakes are surprisingly good nirmally. the last few days i have been experiencing a strange issue where the brakes work normally for 5-10 operations then all of a sudden the pedal will press to the floor and hardly engage. I thought my fluid level would be low but its fine, its not loosing any fluid and vaccum seems to be working normally. i have checked all of the wheel cylinders, no leaks. and the flexi hoses are fine. Im pretty much out of ideas. Has anyone got any inspiration??

Thanks in advance.

Simon

sounds to me like the main pressure seals are giveing up the ghost in your master cylinder wich may need replaceing

good call, the reservoir is pretty black inside, although the fluid is clean, could be a sign of the perished rubber. think they are about 30 quid so ill give that a go tomorrow.

many thanks