[spdbrake]

The bleed nipple won't suck air in. There is no check valve in it. It seals at the conical taper at the base, not allowing to fluid squirt out when the the pedal is depressed.
It can only Suck Air if: You release the pedal while bleeding before the person under the car tightens the bleed nipple.

The Speedway Slave will probably work. I'd try and stick with the same Bore diameter you currently have so there no chance of running out of stroke travel.

If you choose a larger dia slave the the stroke at the clutch will be less and then you may have to up the dia of the master to get the clutch to fully disengage when the pedal is depressed.