That's a good idea about a spring loaded check valve, if this is an ongoing thing I will do that. I assume I could then redirect the back flow water to somewhere preferable.
No, the valve does not leak when they are running, it is only after I turn it off that water comes back up.
And yes, I believe I turned the water back on all the way, never hurts to double check though.
I have tried a few sprinkler heads, one of them was brand new.
At the moment there is only one sprinkler head on this line (one of the large rain bird maxi rotor canister types). I am guess there are more connections, they were just covered up. I have a large bed of gravel covering where I think the rest of the line is.
I like your idea to cap off the sprinkler head, and turn the water on, then wait for a complete break. I bet I could then uncap the line, and see a total loss of preassure, indicating a complete break. Then I could know for sure and start digging up the gravel and then dig up the line. Does anyone want 1-2 tons of free gravel