Abstract: A manhole cover for a mobile tank for transporting flammable liquids is provided with a piston cylinder having a spring-biased ball valve having upper and lower seats. The lower seat is slotted to allow flow of non-compressible liquid in both directions when the ball is seated on the lower seat, thereby to allow the cylinder to move in both directions. The cylinder is attached to the manhole cover which lifts to a limited extent and allows venting in response to fluid pressure rise in the tank. Following venting, the manhole cover and the cylinder return downwardly to their initial positions. In the event of a sudden large surge in fluid pressure, due to tank roll-over or other emergency conditions, the force of the ball spring is overcome and permits the ball to seat on the upper seat, thereby stoppping the flow of liquid, thereby preventing upward movement of the piston cylinder, thereby preventing lifting of the manhole cover.