I had the tank and hose. I just had to buy fittings and valve.
that tank is a 5 gallon coleman fresh water tank
made of HDPE plastic that someone tossed. It had a spigot but the spigot was broke. If you can make another setup then a spigot is also a metering valve if you figure out how to attach a hose. For this one, I cut off the broken spigot, tapped threads of 1/4 pipe thread, screwed in a nylon hose barb.
1/4 thread by 3/8 tube nylon barb $1.60
pvc valve $2.65
barb for pc valve 1/2 thread by 3/8 barb $2.00
plastic grey pipe nipples 2 for $1.00
grey plastic tee $1.25
The plastic water well valve is $9 which I did not get yet.
It has 3/4 thread but you can get a 3/4 to 1/2 reducer bushing.
The well valve feels like it has a very very low cracking pressure.
Feels like an anti siphon valve used on gas tanks
So I will test to see if it works perhaps with a dye in the tank.
I had thought maybe a swing check but that would not be right as the
tank water would just flow out of it. Any valve you use needs to hold the tank water in until the head pump creates a suction then open.
Lowes also has PVC check valve backflow preventer 1/2 inch thread I could try.
The weight of the water in the tank will help open the valve.
The well valve is similar to this one here.