It looks basically like this
Code:
-------- EA ---------- EA
| |------\ P | |-------| (plug)
Pressure In-----| DS | EB |--------| SS | EB
| |------/ | |-----> (open to atmosphere)
-------- ----------
| | A| B|
A| B | \__/
------|=======|
Use the DS to control direction, and the SS to allow/disallow movement.