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Here's how I did mine. I ordered 3/8 SMC valves because I wanted mine to have decent speed up and down. I later slowed my dump/down valves down.

If you look, the lower valves are on a T fitting and are the dumps. The ones above it are the fills if you will. When the top valve gets voltage it opens up and pressurizes the air line including the T fitting and the dump valve. When the dump valve gets voltage it opens up allowing the air in the air bag and the line to escape. I am using 8 valves so I can control each individual wheel. 1 fill and 1 dump per wheel. If you wanted to go cheaper you could get away with only 4 where 1 valve raised both wheels and one dumps. Only issue with that is you don't have individual control. I have had leaks in the past where you just need to add air to the leaking bag. Without individual control, you add air to both only if you only need it to one.