I've run across occasional situations where farmers complain that their sprayer's booms drool and dribble spray when turning on headlands or crossing waterways. They've already equipped their spray nozzles with "no-drips," so they're puzzled why the no-drips don't prevent dripping and drooling.
There are numerous possibilities, but one I always check for is air in the spray lines. Imagine the hose or pipe to which the spray nozzles are attached. Imagine that that hose or pipe extends a foot beyond the farthest spray nozzle. When the sprayer is first turned on, any air in the supply hose or pipe gets forced out of the next nozzle in line--except for the pocket of air trapped between the last nozzle and the end of the spray line.
When the sprayer is turned on, that air is compressed to spray pressure--let's say 45 psi. When the sprayer is turned off, there is no longer 45 psi coming from the spray pump, but the air compressed in the end of the line is still at 45 psi. So the liquid trapped in the boom still "feels" 45 psi of pressure and sprays out of the nozzles until line pressure falls below the trigger pressure of the no-drip shut-offs.
Depending on circumstances, that small bubble of air stays in the end of the boom and gets re-compressed every time the spray pump is turned on. To eliminate the stored energy of an air pocket, with the spray tank full of un-treated water, lower the boom to below water level in the tank. Tilt the booms up, then loosen the pipe fitting or pipe plug on the end of each raised boom section. Air will bubble out, and when clear water flows, re-cap the line.
As mentioned, there are other potential causes for drooling, dripping spray booms. But it never hurts to check for air bubbles in spray lines before panicking and trying more expensive remedies.