How do I isolate the leak?

The brake pedal is pretty stiff, but it might bleed off a tiny bit. Not much if any though. No visible leaks. Brake pads are good. I did not adjust the drums, but the discs are not doing their job. It seems to have a similar problem that it did before I started the MC replacement. I used a master cylinder from sumittracing that came up for my specifications. It looks identical to my old one except for in the inside of the MC chambers, one of the two holes is slightly smaller than the old one. Does that make sense?