I have a new plaster pool and they use lots of acid for the first year
or so. I've plumbed a line from the inlet side of my pump to a tank
with 3 parts water, 1 part acid. I'm using irrigation drippers in
line to regulate the flow and have it just right so it keeps the pool
pH level correct. I know of other people who have done this and it
works long term.

My problem is that when the pump shuts off, a small amount of the acid
still siphons into the pool plumbing. Maybe because the tank is
slightly higher than the pool level? Its about a quart over night.

I was thinking maybe a small spring loaded check valve would help,
where just the spring pressure would be enough to stop the siphon
flow, but when the pump is on and creates a vacuum, it would be enough
to move the spring. Any ideas?