X1/9 rear brakes, air leak?

Hi,

My '78 X1/9 seems to be leaking air into the rear circuit. I've bleed the system a couple of times and within a few months the brakes go soggy again.

Can air leak back in through the calipers?

Unlikely that it's leaking back in through the calipers themselves (the piston seals that is)

Does the system loose hydraulic fluid over time? If so, you may have an upstream leak (pipe fitting, bleed nipple etc) which weeps a little under braking, and allows air in the rest of the time.

Otherwise, you may be looking at a master cylinder. Not expensive, but a bastard to fit.