The bench bleeding procedure calls for the outlet ports being plumbed back into the partially filled reservoir. Usually, plastic fittings w/the correct thread are supplied w/the master cylinder that have a nipple that you attach a piece of plastic tubing to. The tubing then goes back into the brake fluid in the reservoir.
EXAMPLE OF PARTS USUALLY SUPPLIED W/A MASTER CYLINDER TO BENCH BLEED
Both front and rear ports are done at the same time.
If you look closely, you can see the lines leaving the outlet ports, looping back into the reservoir in the image below:
Another way to bench bleed the master cylinder that recently came to my attention when I bought a master cylinder for my '80 Malibu, is to completely block the outlets from the master w/plugs. So nothing can escape. Then the piston is depressed until no more bubbles issue from the ports located on the bottom of the master reservoir.
The master cylinder piston shouldn't be bottomed out during either bench bleeding procedure.
Whether or not this will also work on other master cylinders, I don't know.
MAKING A PRESSURE BLEEDER
Even though I have a MityVac, I find it too small, volume-wise, to be a very convenient way of bleeding brakes.
It's actually easy to make a pressure bleeder from a garden sprayer. Some use air pressure to push the fluid from the master cylinder reservoir through the system. This requires you to keep a close eye on the fluid level so it doesn't run out during the procedure.
Another way is to use a small volume sprayer and pour the fluid into it and pressurize the master cylinder reservoir w/fluid (not air). This means no chance of the fluid running dry- but does mean the sprayer pressure has to be watched and maintained and the master reservoir needs to be drained back to the correct level when done.
I have mixed feelings about saving the brake fluid once it's been poured into the sprayer tank, but I suppose it would be OK as long as the tank was spotless and dry. But not if any old, discolored fluid is allowed to contaminate it- like by siphoning the excess fluid from the master when done bleeding; I would put that into a separate container to be tossed.
I use a rather large (2 gallon) sprayer. This gives a good amount of "run time" before the tank needs repressurized. But instead of pouring the fluid out into the sprayer's tank, I leave the brake fluid in the quart container it came in. I remove the cap, insert the suction hose of the sprayer and that's that. Towels are used all around the quart container to support it and keep it from tipping over.
And if you make a bleeder like this- keep the wand- just unscrew the tip, so you can turn it off and on at will (some tute said to lose the wand- don't!). A couple of the tutorials show the whole wand cut off and I believe that's a mistake.
There are several tute's online, here are a few: