A blow-off valve for a hydraulic dashpot comprising a fluid charged cylinder (1) and a piston (3) that travels back and forth therein on the end of a piston rod. The piston partitions the cylinder into two compartments (6 & 7) and is provided with breaches (4) and shock-absorbing valves. The piston rod (2) or an extension (8) thereof is hollow and accommodates operating components (13).