I suspect the pump check is a membrane type silicone valve also - I will pull one apart to have a better look but I have attached a drawing and specs - They are a positive displacement pump with two tiny pistons that operate on an eccentric. They are definitely PD as if you plug the outlet it will stall the pump. I think that's why they are working well here as speed is very linear to output . They do make them for moving water but I suspect they are not up to "chemical" use - maybe glow fuel? When I get my cheap ones from overseas I will sacrifice one for the cause. Im using Amazon supplied ones right now and they are $10 a pop (vs $1.50 direct)

https://www.maxcleversz.com/html_pro...r-pump-94.html