An arrangement for applying a back pressure to a shaft seal of a mechanism such as a pump comprises a piston slidable in a cylinder one end of which communicates with the fluid being pumped and the other end of which contains oil communicating with a sealed chamber on that side of the seal remote from the fluid. Hitherto, the piston has created a preponderance of the back pressure over the pressure of the fluid by virtue of its weight. This preponderance has been very small, and the rolling diaphragm seal needed on the piston has been easily ruptured if the oil has been lost, thereby allowing possibly hazardous fluids to escape. A greater preponderance of the back pressure, allowing the use of a stronger type of piston seal, is provided by urging the piston towards the other end of the cylinder by a spring as well as by the pressure of the fluid. A magnet on the piston remotely actuates a series of flaps hinged along the exterior of the cylinder to give a visual indication of the position of the piston and also effects automatic replenishing of the oil if necessary or operates a remote warning signal when replenishing is needed.