I finally found out WHY only a cup of water leaks out if you decide to
remove the drain valve from a hot water heater in order to replace the
nylon valve with a brass ball valve.

This recent OSH article explains the proper procedure to create a vacuum
such that only a cup (and not all 50 gallons) of hot water leak out when
you remove the drain valve at the bottom of the hot water heater tank.
http://www.osh.com/Cultures/en-US/Ar...aterHeater.htm

Specifically, the article says
" When replacing a water heater drain valve, turn the gas control to the
"Pilot" setting or, if the water heater is electric, turn the power off.
Connect a garden hose to the existing drain valve and drain the water to a
safe location until the water is warm. If the valve is inoperable, you
should run hot water through a large tap, such as a tub spout, until the
running water is at a warm not hot temperature. Allowing the water to cool
to a warm temperature will minimize the chances of you getting scalded,
when you replace the drain valve.

It is not necessary to drain the water heater to replace the drain valve,
as long as the pressure is off. To temporarily disable pressure close the
shut-off valve on the cold line and open one hot water faucet. Water will
run for a while and then stop. Once water has stopped, close the faucet to
prevent air from entering the system. Warn others not to use the water
supply, while you are replacing the valve, otherwise you may get drenched.

Expect some water to leak from the tank, when you replace the valve (maybe
a cup or so). When the job is completed close the hot water faucet you
opened and open the shut-off valve on the water heater cold line.

A good choice for a replacement valve is a 3/4-inch ball valve. You will
find ball valves, plastic-lined steel nipples (if one is needed) and
adapters to go from 3/4-inch pipe thread to hose thread (which is needed to
be able to attach a garden hose to the ball valve for flushing and
draining) in OSH's plumbing department."

Donna