Throw the vac thing in the trash where it belongs. This is for you and the OP.
To bench bleed a master perfectly every time and at the same time diagnose its functionality.
Put master in a vise clamped by one of the mounting flanges only. Do not clamp around the bore.
Acquire two plugs for the ports.
Insert plugs finger tight.
Fill master with brake fluid from a sealed container. (that doesn't mean a fresh bottle, just one that has been stored with the cap tight)
Stroke push rod in and out a couple of times.
Open front plug like a bleeder and while open, push the rod in all the way and hold it. Close plug and let rod out slowly. The vacuum created will pull fluid from the reservoir into the bore.
Continue until you push in on the rod with the plug open and fluid comes out around the threads. Close plug while fluid is coming out and make sure it is tight enough to seal.
Move to back port and repeat.
Once you have fluid at both ports and sealed, continue stroking the pushrod in and out until you get to the point that it will only go in about 1/8-3/16". It is now ready to bolt onto the booster.
Now for the diagnostics part. A bad master will not bench bleed correctly. It won't get to the point where you can only push in the rod the small amount. It will bypass fluid around the seals and you can feel that happening. You can not tell this with any other bleeding method like the little hoses, vacuum bleeders, or power bleeders. You can only tell this by manually bleeding with the above method. It is no fun installing a bad master after bench bleeding and doing all that work to only have to do it again or chase your tail thinking it is something else because you new out of the box master is bad. BTDT and it sucks.
Thanks for this instructions/tips.