One way valves

[Kevin Thorp]

This is the pneumatic symbol for a flow control valve.

Picture your solenoid valve above it and your cylinder below it. Air flows in freely through the check valve on the right, but when the solenoid valve shifts and the the air tries to exit the cylinder, the check valve stops it, forcing the air through the adjustable needle valve on the left.

Cylinders typically have a flow control (A.K.A. speed control) valve on each port; one controls the extension speeds and the other controls retraction.

They're great for reducing damage to robots and fingers!