[Sirrotica]

Jack, once it gets over the highest point it runs downhill same as any siphon does. Just bumping the pedal at the very top of the travel will pump fluid to the highest point. Once it breaks over the top of the highest point and travels below the level of the brake fluid in the reservoir the siphon will continue. I've never seen any car or truck that when the bleeder screws are opened that fluid doesn't run out, so once the system is filled with brake fluid it's going to run out of the bleeders causing a siphon.

I mentioned letting the wheels hang, with stands holding up the frame. If you wanted to tip the chassis end to end I'm sure that physics would work just fine too, I've never tried it myself because I've only worked on a very few cars and trucks with the master cylinder under the floorboards, one mail Jeep back over 35 years ago is one that sticks in my mind. Basically you have to know how a siphon works, and bump the fluid with the short stroke at the top of the pedal travel to start it. Once the head of the fluid is below the level of fluid in the reservoir, nature takes over.

I believe most people have difficulty gravity bleeding because they don't know enough to use the short strokes at the top of the pedal travel to move fluid over the high points in the system. Once fluid is dripping out the bleeders, the tough part is over. You just wait a minute or two and the system is bled and you just stood around and watched. 99 times out of 100 you have a nice firm pedal the first time, no residual air in the system. Nothing to hook up, no special tools, no begging someone that can't follow simple instructions to assist, no matter how simple (think your wife, that doesn't like the smell of the garage anyway). It's quick, and almost always works the first time.

One other problem I've seen people make when just changing a master cylinder, or replacing a section of line, is bleed the system at that point. You don't have to go all the way to the wheel cylinders or calipers to get air out. There is only air in the part of the system that you worked on. I have replaced a section of line running to the rear, and gravity bled the system at the flex hose so I didn't have to deal with the wheel cylinder bleeders. Trying to open rusted bleeders and breaking them off is unnecessary. Again just bumping the pedal at the top of the travel will let air escape back into the reservoir. You'll see bubbles of air because air is lighter and it will seek the top of the fluid column. You do have to lightly stoke the pedal to uncover the ports by moving the spool, and let the air out of the lines. It's all physics, and no one can change them.