Yes the used master could be the problem .
One thing you could try is to take out the master ,reservoir ,and the line going to the slave.
GENTLY ,again , GENTLY , mount the master in a vise with the actuating rod pointing down.
The reservoir must be held above the master cylinder.
The slave line has a check valve in the end that connects to the slave.
Push the actuating rod into the master , at the same time depress the check valve in the end of the line on the slave end (use a nail,screw,etc).
Release the check valve.
Then release the master actuating rod.
Do this 6-8 times.This will bleed all the air from the master.
DO NOT LET THE RESERVOIR GET EMPTY.
Reinstall the master and you should be good to go.