[Sirrotica]

Or if it were in the car, the weight of the column of fluid in the lines will pull fluid from the reservoir as in a siphon. There is an approximate 2 foot drop from the firewall to a hanging differential with the wheel cylinder.

Obviously my years of experience having successfully done what you allude to as impossible, carries no weight. I couldn't even begin to count how many times I have gravity bled drum brakes successfully, most of the cars likely had residual valves in them since not just GM used them. The readers can make up their own mind as to whether it will work or not.

Just to further confirm, about 3 months ago I broke a rear brake line while changing the rear wheel bearing on my wife's 2008 Vibe, which has a drum disc combination. I had to make a new line and install it. Of course I then had to bleed the rear half of the system by gravity. Hard to believe for the doubters, I successfully bled the system the same way I've been doing it for 50 years. Hands on experience again, the process works just as described.