Sure, the sentence is correct......! You may be asking how ? I will Explain bellow:
1. First let's see what is the meaning of the word "DRAIN"
cause the water or other liquid in (something) to run out, leaving it empty or dry.
"we drained the swimming pool"
2. Still wondering that this word "DRAIN" will suit only with liquid substances right ? Here follow the explanation.
In normal English the usage of the sentences like drain the gas tank of your car, drain the leaking cylinder are common because gas is
stored in its liquid form when it is stored inside a highly
So there is nothing wrong to say like:
a. How to Drain a Water Heater ?
b. drain gas tank
c. drain gas from boat tank
d. drain gas from snowblower....etc.
3. Still confused ?
you can google the word "Drain gas..............." You will get a thousands of sentences which uses this phrase.
Hope your doubt is solved and question is answered.