From the Apollo 12 Lunar Surface Journal:
110:33:41 Bean: I got the 413 in and cycled the Parker valve.
110:33:42 Conrad: Okay.
[Jones - "What's a Parker valve?"][Conrad - "That's one of those shut-off valves for the helium pressure."]
[Jones - "And 'recycle'?"]
[Conrad - "We probably cycled them (turned them on and back off) to make sure they were off."]
[Frank O'Brien - "A recurring problem in the Apollo program was that the RCS propellant isolation valves - essentially, shut-off valves - occasionally closed during periods of heavy vibration or shock (such as during a landing), rendering that particular RCS thruster unusable. Because the Service Module and Lunar Module used similar components, the valves in the RCS systems of both spacecraft could be affected by this. Indeed, starting with Apollo 12, one of the first post-landing steps for the Lunar Module was to cycle these valves to ensure they were open. Fortunately, this problem usually was regarded as a nuisance, since it was easily remedied."]
[In 1999, Journal Contributor Neville Kidger notes that the valves were made by a U.S. unit of his employer, Parker Hannifin PLC. "Al Bean first referred to cycling a Parker valve on Apollo 12 just after touchdown and at least one of our employees, Anthony Tonge, remembers that from the real time audio!"]
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