Landscaping - Irrigation Line Leak

I have a 7-zone underground PVC irrigation system. Six of the zones are valved on an automatic timer. The 7th is a straight pipe out to a garden hose bib. All of the timed zones and valves work normally. However one line has a slow leak; small but over 100ccf per month. How can I find which line it is? When I turn on the main supply valve, I can hear the rush of water building up pressure (bled off because of the leak). With a mechanics stethoscope, I can hear nothing in any of the lines in the valve box. I suspect a damaged PVC pipe or connection, perhaps due to winter freeze or tree roots. There are no wet spots discernible in the yard and the sprinkler heads do not dribble.

PVC stretches and can sometimes come out of its glue fittings or if they used cheap glue, it can fall apart. You could also have a leaking valve, cracked valve, loose fitting somewhere.... basically anywhere in your yard.

To find this you will have to be patient and use a little luck.

1. turn the clock to off but leave the water on.

2. since it is so slow you will need it to surface. Since this is small, it will take a long time to show up. Depending on your yard, this may never happen.

If none of the heads dribble then the leak is before the valves for the individual zones for sure. Without knowing the layout of your system it is hard to know how to troubleshoot it. If it were me I would inspect the inlet side of all the valves for damp areas in the soil around them. If you don't notice anything there it may just be a waiting game for the leak to get worse and cause a wet spot in the yard. If the faucet in the garden is PVC pipe I would also dig around the base and check for leaks in the joints there. faucets tend to get yanked on by pulling hoses and PVC doesn't take kindly to that.