sometimes pushing the slave piston/s all the way in while the system is open helps to bleed it right the first time.

or perhaps-
the slave piston seal can get dirty so you need to gently remove the piston seal and clean behind it. anyway its a good thing to do. with the system open  the piston should slide in with the force of maybe two fingers. if its sluggish and stiff something is up.

you can even polish the barrel a little before putting the slave piston back in.
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i doubt the slave has anything to do with the problem
my best guess is the master.... either the your replacement seal is incorrect or that it was mushroomed backwards upon installation.  so double check the work you have done allready. maybe its a pet hair bisecting the seal you installed...