Back in the old days when I was a kid and couldn't afford any new parts I had to rebuild everything
So I did my share of brake masters and figured out how they worked, wasn't magic anymore, lol
And the same for wheel cylinders, no such things as calipers in my world at that time, lol, drums all around
So I would have to pull off a wheel cylinder, hone it, put in new seals as needed and reassemble, so brake lines were disconnected for awhile
I found if I used a stick against the seat to press down brake pedal a bit that would close off the Master cylinders reservoir, kind like putting your finger over the end of a straw so fluid doesn't leak out
Really slowed down the leaking fluid and of course plug the hose as best you can, lol
Gravity bleeding got the air out usually
If it didn't I built a "power bleeder", well kind of
Block of wood with an air valve added, valve part removed, 2 C-clamps
Inner tube, yes the thing that used to be inside tires, lol
Would use a hand pump to pump up the inner tube to 5 or 6psi, not much
Then put a hose between the two
Power Bleeder............lol
Could usually do all 4 wheels without adding fluid or pumping up inner tube again
Mom was my "power bleeder" for awhile but told me to "figure something else out", so I guess she invented it
Like they say "Mother is the necessity of invention", or something like that