The master cylinder is probably bad. It's very common to replace the slave cylinder and have the master fail shortly thereafter because the master cylinder's diaphram is weaker than the slave cylinder's due to age. It usually leaks inside the cab, on the firewall above the pedals. Sometimes is doesn't really leak much, but the fluid still isn't moving much because of the weak diaphram.
Your symptoms sound identical to mine and about 3 others that I know of who all had the exact same sequence of events. Each time ended up being the master cylinder.
I found out later that you can buy a pre-bled master/slave/line combo that replaces the entire thing for about the same as replacing those individual components. They all fail about the same time, and replacing just one typically causes the others to fail.
