I recently came across this list so add it here in case it helps someone. The ring sizes and comments are not mine as I have no experience of this pump. Might help though ... I have added info to the O ring database.

"O" rings and part numbers for the FX "Third Generation" pump
U-006 Urethane 90 durometer Part number 30
BN-009 Buna Nitrile 70 durometer Part number 40
BN-106 Buna Nitrile 70 durometer Part number 39
BN-117 Buna Nitrile 70 durometer Part number 35
23 x 1 mm Buna Nitrile 70 durometer Part number 25
2.5 x 1.5 mm Buna Nitrile 90 durometer Part number 38
9.3 x 2.4 mm Buna Nitrile 70 durometer Part number 27
9.3 x 2.4 mm Buna Nitrile 90 durometer Part number 37
11.6 x 2.4 mm Buna Nitrile 70 durometer Part number 28
21.3 x 2.4 mm Buna Nitrile 70 durometer Part number 36
18 x 2.5 mm Buna Nitrile 90 durometer Part number 24
20 x 2.5 mm Buna Nitrile 70 durometer Part number 31
The first four are Standard "O" ring sizes. The next two are regular Metric as well as the last two. The others are Japanese Metric, hence the 2.4 mm cross-section size. It is very important to install parts numbers 35 and 31 in EXACTLY the right sequence, or your pump will not operate properly. Lubricate them while installing and afterwards with diver's silicone grease. Do not use any petroleum based greases, PERIOD.
I could not get part number 37 in 90, so I used 70 in its place. It might not last quite as long, but until I can get the proper one, it will do. The most important one to get as shown, is the very first one. If you get anything else, it will blow out real fast, like in two or three fillings. Trust me on that one.

Cheers, Phil