[AT] Lapping Valves for Air Compressor

[Sheppard, Charles E]

Has anyone experience in lapping air compressor valves?  I'm trying to
recondition an old Westinghouse Air Brake compressor that has ring-type
valves.  They look like a washer, spring-loaded, that faces against a
seat that has two raised ridges contacting the inside and the outside of
the washer.  What I have done is lap (sand) the pieces on a piece of
plate glass with 320 grit, then 600 grit W/D paper, then spot them on a
surface plate with Prussian blue.  Well, I have done that but I don't
see any spots.  I tried to rub a fairly thin coating of blue on the
washer-like piece.  Either it is flat within 1/10 or I'm spotting it
wrong (too much movement, too much pressure) but it comes off the plate
clean.  I've never done this before so I don't really know what to look
for!  Should the low points be bright blue?  Appreciate any comments?