[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.

[Schurkey]

Originally Posted by ftwmlp

As of now master cylinder is bone dry again, suspect we never had the lines full after all,

I'd be looking for leaks rather than a different bleeding method. How long did it take to empty the master cylinder reservoir? DId you check the wheel cylinders for leaks? Can you see the brake tubing and the flex hoses?

This truck is not the greatest candidate for gravity bleeding, since the master cylinder is mounted low on the frame rail instead of high on the firewall. Lower head will make the siphon effect more lazy.

Can you get the master cylinder tipped "down" in front? Park on a hill, jack-up the back end, something like that? DON'T disconnect the brake tubes.

Fill the master with fluid. Tickle the brake pedal--air in the master should escape to the reservoir. This can take awhile.

Once there's no more bubbles in the reservoir, bleed the brakes just like you did before. Given a choice, I'd prefer to pressure-bleed. Most folks won't spend the money for a decent pressure bleeder, though.

I gravity bleed for minor work at or near the wheels--most brake work; but generally prefer to pressure bleed for major work, or work at/near the master cylinder.

[ftwmlp]

Thanks for the reply. We only go down to the property where this beast lives once a week most of the time, less in the winter nor do we jack with it every time we are there. Don't really know how long it took master cylinder to empty, but never saw leaks enough on ground under wheels to trigger investigation. Yes we can see and get to all the bleeders, tubing, and flex hose.

It's interesting you mention getting the master cylinder "tipped down" by raising the rear. In one of my conversations with a man at the remanufacturer, he told me a story about one time having to use a fork lift to lift the rear of a similar truck with a forklift to get a good bleed job. Like the wise old man I am, I dismissed this as a BS story since that is opposite of all my limited experience. Just to be clear, this truck has a "traditional" master cylinder mounted on the firewall, it is about chest high on a normal man. There is some kind of one way valve first downstream from that then on to the frame rail mounted "booster" (all flex hose) then out bound in hard lines to the front and rear wheels (dual). I have access to a pressure bleeder but having never used one could not assess the pros and cons of using that method.

Again, I appreciate you taking the time to offer your help.

Mike Pearson

[Schurkey]

Originally Posted by ftwmlp

It's interesting you mention getting the master cylinder "tipped down" by raising the rear. In one of my conversations with a man at the remanufacturer, he told me a story about one time having to use a fork lift to lift the rear of a similar truck with a forklift to get a good bleed job. Like the wise old man I am, I dismissed this as a BS story since that is opposite of all my limited experience.

Air pockets rise in brake fluid unless prevented by fluid viscosity or surface tension.

Most master cylinders are mounted to the vehicle with the front tipped "up", to align them with the pushrod from the brake pedal.

Any air trapped in the master cylinder goes forward--"up"--AWAY from the drilled passages leading to the master cylinder reservoir. Finding a way to tip the front down allows the air pocket(s) to move toward the cylinder/reservoir drillings, and applying just a hint of pressure will blow the air from cylinder into reservoir. Pumping too far will move the master cylinder pistons enough to cover the ports--so you just barely tickle the pedal. Maybe the master cylinder pistons move 1/8 inch or so.