You want to produce the pressure and you want to switch the valve by motor. With my idea you can do both with a single motor. This can be easily controlled by RCX or manually.
Compared to the 1st version which is nice on mobile robots this 2nd version is more powerfull, but larger. For stationary use is may be the better choice.
Click here to see building instruction.
CAUTION: You
should not run the motor all the time without consuming pressure. This
could damage motor, pump and gearing.
The basic machinery look like this:
This is a side view:
Motor and airtank is removed here to have a better view:
Have a lot of fun.