Not sure why there is endless air coming out of the lspbv, but in my endless brake bleeding hours on the 80 and gallons of fluid, the technique I found worked the best was this.
Using a motive, put 10 psi on the system and open whichever valve you want to bleed. Then with that valve still open, go stomp on the brake pedal pretty hard, but let up on the pedal VERY slowly. Repeat a number of times. Even with that 10 psi on the system you don't want to draw any air back into the lines from the bleed valve so that's why I didn't let up at normal speed.
I did this with two people (had an assistant pound the brakes, and let up slow) while I watched the fluid come out. None got sucked back in, and the sudden increase in pressure really forced some air out that just the standard Motive or standard "pump 3 times, open valve, close valve, repeat" method didn't get.
The benefit to using the motive is you can already have the valve open and fluid moving when you stomp on the pedal. Suppose you could do this just with gravity bleeding method too.
I do recall that I got quite a burst of air out of the lspv, but it only went for about 5 seconds and then fluid came out. Unless the valve is bad or the system has a leak somewhere, can't imagine that more air is getting into the system??
Though if you did softlines, hardlines, calipers, etc I would imagine it took me a whole lot more fluid than 2/3 of the reservoir twice to get the lspbv bled correctly.
2 cents from a noob.