Superb Clutch Pedal

Hi

The clutch pedal has been playing up.

When leaving overnight it drops right down but can be put back in place by putting toe under it to bring it back up.

Fluid levels up but mechanic decided to change master sylinder (110,000 miles).

This has been okay for about 500 miles but is now doing the same thing.

Any ideas on this before I take it back?

Thanks