F1 truck MC bench bleed

[3twinridges]

I have read about bleeding the master cylinder on the bench. I thought I would try this with a new Wagner MC I bought. I was going to assemble it complete with the three way fitting and stop light switch. Attach a couple short flared brake pipes to the fitting to bleed it. How do I keep from reintroducing air into the cylinder when swapping the short bleeder pipes with the actual brake lines. Seems like the fluid would leak down and air would be introduced making the whole process a waste of time?

JB