[Andrew Schuetze]

Originally Posted by Tank2010

is it against the rules?

Not legal and please at least download and look at the competition manual before posting and asking if it is allowed.
Here is a link to the pneumatics manual if you want to learn about how to incorporate them into your robot. And from the Robot Manual, see section 8.3.9 Pnuematic system
specifically rule

<R71> To satisfy multiple constraints associated with safety, consistency, robot inspection, and constructive innovation, no pneumatic parts other than those explicitly permitted by the Pneumatic System Rules may be used on the ROBOT.

And

<R72> In addition to the items included in the KOP, pneumatic system items specifically permitted on 2010 FRC ROBOTS include the following items. All included items must be "off the shelf" pneumatic devices rated by their manufacturers for pressure of at least 125psi, and used in their original, unaltered condition (except as required for assembly with other components).

A automatic water sprinkler value is by definition not a pneumatic device.