Syder, you're asking the same question over and over again, if I answer it, will you just ask again? lol... Seriously, in your case, just go for plastic in every part you can, as it's cheapest, and will not corrode or react or be toxic to plants. STC does make good valves, the plastic are apparently good, for 19 usd, and stainless is about 35 usd. There are some blue cheapo ones on ebay for like 15 bucks, and they'd probably work too, but who knows it they'll take the beating of operating hundreds of time per day for months and years in a row? In the end, this is just a very small apsect of all the thinking you'll have to do to get this thing up and running right, while it's good to be detail oriented, you just have to move on, make some choices, and understand you'll likely change out some parts as time goes by in your pursuit of perfection. Building an HPA is likely more for people who enjoy engineering and making their own setups, not so much for people who just want to scavenge information and copy without really understanding what it is they're actually doing. Of course this is where alot of us noob HPA people run into issues, because we attempt to deviate from the proven setup too much- so there is a balance, at least in the beginning. My personal path was to try to understand and copy as much as possible people like G-Love, Atomizer, and tree farmer, because they are having good results. From here I can try to test out new ways and put my own personal "spin" on things.