Changed Slave Cylinder, Now Cant Get Lines To Bleed

[Rotary Evolution]

uhhhh, you're friend who is a "mechanic" just randomly threw out that it needs a slave? your friend isn't much of a mechanic...

was the slave cylinder leaking? if not, then it probably wasn't your problem, because a slave cylinder is a simple plunger and it has 2 failure modes, seized(which would still give pedal pressure) and leaking externally(which it would still work but it would cause the pedal to fall and empty out the reservoir in no time).

your problem was likely a faulty MASTER cylinder, because when the master cylinder fails it usually bypasses the fluid internally back to the reservoir. when it fails in that manner then you won't have any pressure working in the system, hence why you can't bleed it.

there's nothing worse than techs who can't actually diagnose a problem but simply throw parts at a car until they get lucky. ALWAYS check for leaks first, if nothing jumps out at you then it is probably your masters.