what year, tranny you got?
external slaves are all one unit from master to slave, (you can find separate master's and slaves, but are not easy to swap)
buy the whole assembly, already pre-bled with fluid in them (they are that much of a PITA to bleed they come with fluid in them from the supplier)
short of the cap being bad there should be a telltale sign of fluid somewhere on the master, the line or the slave.
If it is low on fluid and just sucked some air into it and not leaking...take the slave off the tranny and with a helper up top watching the fill res., pump the piston in/out until no bubbles are present up top, filling if necessary. Don't use the clutch petal, it wont work that way...you cant bleed these top down, you have to use the slave to push air up and suck fluid down. Hold it in a way that the line cant trap air either..it has some 180's and preformed curves in it.
first and reverse sounds like syncros though...you'd experience a bad slave in every gear...although a bit easier since you could be floating, or "power shifting" those gears at speed