slave or master cylinder,- clutch

[nodeward]

So I had the pedal drop to the floor the about 6 weeks ago, swapped the master in the hope of not having to drop the gearbox and it bled up ok and been fine since until yesterday. Pedal dropped again, small loss of fluid but no obvious leaks. Clamped the pipe at the gearbox and can't get a pedal back which suggests a failed master again but this seems unlikely. Is there a good way to bench test the master to confirm before I bite the bullet and get the gearbox out? Or is this common that the slaves fail gradually?