About 4-5 months ago I bought a 90-91 Civic EX 4Dr 15/16" brake master cylinder from one of the members on this board (along with brake booster and 40/40 prop valve) and i believe its junk. I have good pedal pressure coming to a stop, but slowly the pedal drops to the floor while sitting at the stop. No fluid is leaking from anywhere, so what I am thinking is the internal seals are allowing fluid to move past the piston. Anything anyone can recommend to check out first?

 

have someone press the break in and look at the brake booster and see if any air is getting out, had that happen on my 91 4 door. just a bad seal

 

whe i did my rear disk conv i had a similar issue.
as you said it might be the seals of the pistons but i never heard of it going bad
in my case there was just air in the lines and a small vacuum leak in between the master cylinder and the brae booster. check that. did you touch the brake booster? kuz if you did make sure you have the gasket that goes between it and the fire wall

 

Yes, I replaced the Booster with the Master Cylinder... will check for vac leak again.

 

sounds like a bad master C.
Dont be skimpy on brakes.. go buy a new one