A few thoughts on your brake problem:
1. You may have put in the piston cup(s) backward.
This would result in improper sealing effect as you press the pedal.
2. The joints where the lines join the master cylinder body are
improperly tightened-either too tight or too loose- either way
creates a bad seal.**Use a flare-nut wrench to tighten these unions.**
A regular open end wrench may round the flats before achieving a
3. There is still air resident in the master cylinder; it wasn't
completely purged of air when you (presumably) bench-bled it.
This is unlikely but possible.
4. Always bleed brakes starting with the bleeder farthest away from
the master (The passenger side rear), and work toward the master.
I.e.: (1) Passenger side rear (2) Driver side rear (3)Passenger side
front (4) Driver side front.
5. Spontaneous leakage of the system downstream of the new, more powerful
master. Check for a drop in fluid level.
6. A leak within the master cylinder secondary to an imperfect bore.
If in doubt, never rebuild, replace. These are your BRAKES. Your life
and the lives of others depend COMPLETELY on your skill. If you are not
a trained brake technician, don't jack with a master cylinder rebuild.
People can die.
Hope this helps,