There are a number of potential failure points which are downright sobering. No check valve at the mask would be an easy place to start criticizing. However, I admire the guys' creativity. He did this with little more than a junk A/C pump
, piece of plastic pipe, rebar, etc. Brilliant!
As for your question, sliding piston pumps can run quite clean. I write "can" because it does not look like this guy disassembled the pump
to really get the internals absolutely pristine. There's no way I'd want to suck on that without totally tearing down the pump, a filter on the intake, splitting some hose to know the interior
isn't crumbling, etc.