Brakes

[BlueKSO]

I seem unable to bleed the rear brakes on my 63 Flavia Coupe.

Using both pistons on the reservoir of the master cylinder, I can bleed both bleed nipples on the slave cylinder.  The front piston then bleeds the front brakes successfully, but the back piston is solid, will not push down and will not bleed out the rear brakes.
The rear flexible hose has been replaced as it had partially collapsed.  Before I connected up the flexible hose I tried to bleed through with no joy.  Both rear calliper bleed nipples are clear.  There seems to be some malfunction with the servo/slave cylinder. 

Any clues would be much appreciated.

Cheers,  Peter