renault clio mk1 phase 2 1997 brake problem?

hi i have a problem with the brakes on my clio and was wondering if any1 can help.....firstly i had problems with the peddle going striaght to the floor so i checked the fluid which was at correct level then the front calipers and pads plus brake lines and everything was in order so then checked the rear drums and noticed both wheel cylinders wer gone so replaced both cylinders and bled them and re'adjusted the shoes correctly, following this i took it for a test drive and it was ok for about 10 minits then peddle went straight to the floor again but after a few pumps its back 2 normal again then straight to the floor after a short while....would be greatfull if any1 could help ????? many thanks