Seakrakken is unfortunately too accurate for sports RBs as they stand just now, but Firemandiver in Post 3 is right.

Take PVC to start with. Put it into a chamber with a window in it. Blow down, bring it back to surface. Watch it smoke on the way back up, especially with helium: literally the gases that come off it often look like smoke (I must post some pics!). Now try to google something like "PVC Greenpeace" and then "MSDS PVC", and look what you are breathing in. Nothing to worry about right now, but it will not help you attain much in the way of old age. Now take valves that stick, handles fall off, hoses fall off, WOB is through the roof, mushroom valves that stick, or get trapped in the mushroom's spider, mushrooms that soften in water or oil then fold up, etc.

Learn from the experience Paul Raymakers is kindly reporting free of charge on this forum: he has more resources than most home builders, designed his rEvo, improved it, thought he got it "right", then went and tested it. Look at how many mods he is doing from the test results just to get to the "minimum" safety level of EN14143. Until you test it scientifically, thoroughly, you do not have the faintest idea what you have done. Look at the recall of the EDO4: after that mouthpiece killed one or two people.

It is your life, but I have stopped diving things that come from tiny design shops, no matter how big their factor is, at least on my main breathing loop. I put that down to experience.

Home build is one thing, but best to start by building and proving your test equipment, then design your accessory or Rebreather, IMHO.

Alex