On the initial hookup of the water supply to a newly installed piping system, all the lines should be thoroughly flushed to remove any particles of dirt or debris, including solvent glue and thread seal that may have fallen inside the pipe lines during assembly. If the lines are not flushed, then these particles will be carried into the valve, where they may become lodged on the seal and cause a leak or constrict the water flow.
To flush the line, open the drain valve at the end of the line and allow the water to flow out for several minutes into a bucket. Repeat this procedure for each drain valve in your system. Start on the lines that are closest to the water supply and work your way to those farthest away.