2006 Kia Picanto lose brake compression

I have a 2006 kia picante zap, every now and then perhaps once or twice a week I lose compression on my brake pedal thus no brakes. if I pump it a few times it returns. I have had a new master cylinder fitted but this still occurs,if brakes are bled I have no trouble for a few weeks but then the problem starts again. any ideas of what may be the course many thanks