Do I need a new Master Cylinder for motorcycle?

I have a 1981 Yamaha XJ650 Maxim. I recently lost all pressure in my handlebar for front brakes. I tried bleeding it and have taken all the air out of the line. I flushed the fluid and still no restoration of pressure. So i take the Master cylinder apart, clean out the system and also check the rubber seals, everything looks fine. I put it back together and after flicking the handlebar, i see fluid out the top and gradually about 30% of my pressure and brakes are back. However after an hour the pressure is gone and im left with only 10% of brakes. Any suggestions? Do i need a new master cylinder or may it be a caliper problem? It seems like it wants to work... but something is wrong.