All v plows have relief valves. Tee a gauge in at the junction of the angle cyl and valve. Extend the wing to the straight position, then use a come along to pull it back into the v position. Note the psi required and compare it to specs in your owners manual. Does this happen to both wings or just 1? If just 1 try switching hoses side to side. If the problem follows the hose, you have valve problems. If it stays on the same side, the cyl is leaking by.