So, I have a used Liquidator and I need to replace the check valves. I have seen the original grey ones, the little white John Guest ones, some black ones that Rabbit used, and there may be others. Has any one of these turned out to be more reliable? Is there another better choice?

I've used these SMC check valves with good luck. They have a low cracking pressure and large body.

Dave, Would this be the one:
1/4" FNPT Polypropylene Check Valve w/FKM Seals & 302 SS Spring
Maximum Operating Pressure — 125 psi. Maximum Operating Temperature — 140°F. Seat Area Open Diameter — .438. The 694 Series Check Valve covers a wide range of air and fluid applications. The 694 Check is a compact design with a 1/3 psi cracking pressure, high flow rate, and a positive seal at very low back pressure or slight vacuum.

US Plastics Item 22231 Looks like it is Black. It is the one suggested by Rabbit

That's good enough. I used the 3/8" because I had the adapters and I figured the larger disc surface the better.

I was going to ask about the 3/8." I think I will go with 3/8" as much as possible. Since the Flow gauge requires 1/4" I will have to use 3/8" barbed because I can't find a 3/8" speed fit x 1/4" NPT. I only found a 3/8" SMC needle valve with barbs at US Plastics, so I will use that as well. I know the barbed fittings decrease the flow, but I have not found a way to avoid them on these two parts.