Weird Clutch Pedal/hydraulics

Evening all,

Here's one that's been baffling me and a mate for a while now. It's a 2004 focus 1.6 petrol. A good few months ago I changed a clutch and slave cylinder for him as a seal had burst inside the slave cylinder.

So I changed all three parts of the clutch, refitted the gearbox with new seals and fresh oil - All good, well it was for a few weeks.

So thinking the clutch maybe needed bleeding again I hooked up my pressure bleeder and pushed out a very small amount of air and let a good bit fluid pass. Everything seemed well again, good solid clutch pedal and no problems changing gear.

A couple of weeks passed and the pedal started doing the same thing again. So out with the pressure bleeder again, bled the system but no air this time but the pedal feels brand new again. ok, strange but its good. But this only lasted for another 2 weeks before the pedal went the same way. So I replaced the clutch master cylinder on the pedal (not knowing the age or condition I thought this could be suspect), again bled the system and the pedal is brilliant again.

Unfortunately the car went another 2 weeks before going the same way. My mate has put the pressure bleeder on the system for an hour with no loss of fluid or drop in pressure. But simply pressurising the reservoir returns the pedal to perfect working order again.

Really cant get my head round what is wrong here. Anyone any ideas or come across this before?

Regards,

Stef