Whoa, I think I figured this out after looking at some illustrations.
Listen up people! This could be the source of some servo failures!
Non return valves can be inline or screwed into the servo. Depending on
which servo you have or if it has been replaced, the valve may be missing.
It is necessary as it helps protect the servo. Here's what I think:
The Girling MK2B servo has a non return valve screwed into the servo near
the bottom as part of the hose connection, hence it doesn't need an inline
non return valve like the Lockheed. The MK2B non return valve has an unusual
angled fitting thus it is included with the servo and shows in
illustrations.
The older Girling MK2 and MK2A servos have the vacuum connection at the top.
No non return valve is shown in the illustrations perhaps because:
A) a normal straight non return valve can screw into it and thus it doesn't
come with the servo. or
B) it may have had an inline non return valve, or
C) It may not have had a non return valve to begin with.
D) If it was a screw in valve, a PO replaced it with a hose fitting. I
suspect that happened to mine.
Is a non return valve needed? I say yes. Since the improved MK2B servo has
it and the Lockheed has it, the older MK2 and MK2A servos should have it
too; either a screw in or inline.
I suspect the non return valve keeps keeps gas or gas laden vapors from
getting back to the servo. If gas or gas vapor gets to the servo, it not
only damages the filter but more importantly, gas and vapors could damage
the coating in the servo as well as the leather like seal thus causing
premature servo failure! I note that Lockheed advises putting a bend in the
line to trap petrol. They are concerned about gas getting to the servo.
The inline non return valves are in the Victoria British catalog for $38. I
don't know if there is a less expensive alternative.
Andy
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