Either way.
If you have a booster;
and engine off, your assistant will just have to press a lil harder.
And with engine running, the pedal has a slightly less hard feel to it. Try not to confuse less hard with spongy,lol.
BTW;
One time, as I was looking for the fountain; I lifted the reservoir cap just a lil, at a 45* angle, towards the drivers side fender where I was standing and promptly got sprayed in the face and eyes. Don't let that happen to you.
ALSO
the reason I said to pump the brake pedal three times is because each stroke will send fluid down the line.But when you lift, and press again, because of the air bubble, not all the fluid has time to return, so the shoes remain applied to some degree. By the third pump,a lot of fluid has gone out, and the shoes will be out probably about as far as they can go. And the return springs will be pulling hard. So the instant your assistant lifts the pedal, and the compensating port opens, that fluid is gonna try to make a mad-dash back into the reservoir, which can cause a geyser, 2 ft high! Any air in that system is gonna unload as well, making it worse.
If you have never done this before, maybe I shouldn't have said 3 pumps, lol. Try it with just the first pump, until you and your assistant get the hang of it. Mistakes will be made. With the engine off, your assistant can better hear your screams of panic..... Remember the emphasis on slowly relaxing the pedal; only the assistant can control the geyser by reapplying the pedal; the foot must stay on the pedal, and be prepared to reapply the brake to stop the fountain/geyser. With just one pump;Roiling is normal, a geyser is not. With each subsequent pump, the roiling will turn to an ever growing fountain. But if there is air in the line, anything can happen.......
I haven't tryed this but I suppose; To tame the fountain, you could cut and shape something to partially cover the floor above the C-ports. I seem to recall having seen some later model GMs with a floating closed cell foam patch in their reservoirs.