[ftwmlp]

Originally Posted by Sirrotica

Darby, you pretty much said it.

If you've ever siphoned gas, it works on the exact same principle once you have fluid below the master cylinder reservoir level, your work is done. Gravity takes over from there. The small pump at the very top of the pedal travel gets the fluid running downhill, without drawing in air to the system. Atmospheric pressure, and gravity power it.

When done check to see if you have a firm pedal, if not open the bleeders and let it drip for a few more minutes. The procedure works because air will rise to the top of a fluid column, this bears out when you stroke the pedal while observing the reservoir. You'll see bubbles come up from the bottom of the reservoir if the lines have not been fully purged of air. You actually bleed an empty system from both ends using gravity bleeding. If you don't observe any more air in the reservoir, your most likely done. The rest of the air exits from the bleeders during the siphoning process.

It's a simple principle, yet some people make it overly complex.

As I've learned to expect from contributors to this forum, very helpful knowledge and experience sharing! Just curious if anyone has experience with gravity bleeding (or other methods) on a old (1979) commercial Chevy 3 ton dump bed (C60)? This baby is equipped with what they call a "Hydrovac Booster" gizmo mounted on the frame rail. Changed out to a reman master cylinder (bench bled) and installed a reman version of the booster, bled using the two man method as best we knew how, but cannot keep firm brake pedal. All bleeders were removed and cleaned at the wheels, thought we had good clear brake fluid to each before closing it back up. As of now master cylinder is bone dry again, suspect we never had the lines full after all, and have no idea what is supposed to be happening inside this "booster"
Any help would be greatly appreciated.