You need to find a circuit for a Basic Monostable 555 Circuit as the switch input to pin 2 of the 555 will activate the output high (dwell) time to the noid. From the circuit I have, there is no setting for the low time. Every time you press the switch the high (dwell) signal occurs.
Sounds like your using a Basic Astable 555 Circuit that lets you vary the High time and the Low Time, but how are you connecting the switch.
Please post a link to the circuit that your using.
Astable is nice if you want to use an adjustable resistor to vary the ROF
Mono is nice if you want to use a switch to activate the noid. Becareful if you hold the switch down it will keep the noid/valve open too. Well if the ram has enough force to keep it open.