another option is to have a booster to your valve and run 14.4v on your drills.

This system is used by both myself and G3 in there robots.

We have 14.4v for the whole of the robot and then on a seperate feed from the 14.4v battery we have added a 9v PP3 battery in series giving 23.4v which switches the solenoid. Not had any problems so far.

Regards
Ian