that little 'one way' valve lets fluid exit the chamber but prevents it from returning. This keeps the fluid from being 'sucked' back into the chamber when the pedal is released and the piston returns to its home position, instead it forces the chamber pressure to pull fluid from the reservoir. It also aids in bleeding the air out and doesn't allow the piston to pull air bubbles back into the chamber. When i replace a master cylinder i request a reseal kit which usually includes this valve (on the cat trucks i've done) not sure about other brands.

This valve was fairly common on older trucks years ago but since then most mfg's have gotten away from this valve and the ports are just open now on newer systems.

As long as that valve is closing properly and not damaged in any way the system should pump up and bleed out correctly and easily.

If you have good pedal pressure but still have no brake actuation in the wheels i doubt your problem is in the master cylinder. I would look at the wheels, backing plates etc. In alot of cases i've seen where brakes have good pedal but no stopping power its usually in the wheels, wet shoes, worn backing plates or some type of hardware issue 'in the wheels'.

ok now for a dumb question, does this lift have any type of brake assist valve like a lock valve? 'mico brake lock' ???
just curious and off the wall but i had to ask ;o)