master cylinder shot?

71 duster, originally a slant 6 car with 9 inch drums all around, now with scarebird discs up from and 10 inch drums in the back
(about the only part left over from the stock system is the master)

the main issue is the pedal is not consistent
sometimes i need to pump it once before i get pressure, sometimes it is fine

what is really interesting is when i am at a light, and i keep the pedal down it slowly sinks, like there is a leak in the system (there is not)

now, when i set the line lock (plumbed in to lock the front wheels) the car stays put, forever

whatever is going on, i know i am not leaking fluid


does this sound like a bad master cylinder?
and how bad would it be to replace it?
(ive heard of people "bench bleeding" a MC, but not sure what that means