I have done many tests with drill motors (and still have around 10 lying about or burnt out ). If you want to overvolt to 24v, try to restrict the power going into the drill motor via the speed controller if possible, but no matter what you do with current limiting etc, you must use small wheels if you are to go to 24v. Anything over 70-80mm is too much and will strian the motors too much.

I wouldnt advise running on 24v at all infact but you can do some things to reduce the chance that they will blow.
- Make sure all air vents to the motor are open and that air can travel freely around it.
- If possible find a heat-sink from RC cars/planes to fit the motors (there are some sizes that would do)
- Use the correct amount of limiting from the controller.
- Use smaller wheels, around 60-80mm should be reasonable

How much current does the valve take? As you could use a regulator to give the drill motors about 18v, which would be fine for running, and give the valve 24v (only using one 18v battery).

Cheers, Ewan
http://www.micro-maul.co.ukwww.micro-maul.co.uk