meltzie

I have a leak in an underground sprinkler pipe. It's only in one station and the only evidence is that when that particular station turns on there is an incredible drop in pressure. This station has sprinklers running along the perimeter of our property. I cannot find any other visible evidence of this leak, only the drop in pressure. Other than digging up all the way along this station I can't think of any other way to detect the location of the leak. Any ideas? I've already checked all the sprinkler heads on this station and also replaced the valve recently. Thanks.