The first thing that comes to mind is something that has previously happened to me and other owners. You replaced the master cylinder so it now has very tight seals and is delivering maximum flow and pressure. It is delivering this maximum flow and pressure to the old slave cylinder. Just as the master cylinder seals had failed and gotten weak with age (likely leaking internally), so will the slave cylinder seals. The previous weak master cylinder did not overpower the weak slave cylinder seals, but the new master cylinder may now be causing an internal bypass within the slave cylinder.
Second issue is the potential for unknowingly changing the pushrod length when you replaced the master cylinder. Minor differences between old and new master cylinders, and pushrods if they were also changed, can greatly affect the performance of the clutch hydraulic system.
The previous advice given is quite valid, the only definite thing at this point is your system needs some tender loving care.
Good luck,
Larry