At our construction jobsite we have approx. 1,000FT of 8" fire water from the city main running into our existing building. The existing line was insatlled in 1971. We installed a new backflow device as required by the city at the start of the line. The fire department required us to pressurize the existing line to 200PSI for 2hours. We failed. Over the course of 8 hours the pressure gauge dropped from 200 to 50PSI. When we called a leak detection company and they pressurized with helium and air to 110PSI, it held for 26 hours until we had to release the air and put water back in. We tried the water test again and failed. How could it hold 110PSI of air but not 110PSI of water? If there is air trapped in the line when we do our (Hyrdo) water test, could the air be compressed, causing the gauge to drop? I would think it would leak air faster than water, but the opposite is happening. I'm at a loss. Leak detection guy says he can't find the leak if no air is leaking. FD will not accept air test. Thanks.