brake servo non-return valve-necessary?

[andy65 <andy65@snet.net>]

A non-return valve comes for the vacuum line with the Lockheed brake servo
units.
I have the original Girling servo just rebuilt.  Is a vacuum non-return
valve recommended or even needed for the original Girling servos?  I doesn't
appear to have had one.  Are their any specs on these i.e. are they
available at Pep Boys or do we have to get the $39 one from Victoria British
or the like?

Andy