A submersible pump assembly for pumping well fluid from a well, comprising: a rotary pump; an electrical motor operatively coupled to the pump for driving the pump, the motor being filled with a motor lubricant; a seal section located between the motor and the pump and comprising: an inlet port to admit well fluid in the seal section, and a communication passage that admits the lubricant from the motor into the seal section; a flexible compensator element located in the seal section and separating the lubricant from well fluid entering the seal section; a drive shaft for rotation by the motor extending through the compensator element; and a cap mounted around a first end of the compensator element, the cap having a skirt extending radially outward relative to an axis of the shaft to limit expansion of the compensator element in a first direction; characterised in that the first end of the compensator element comprises a cylindrical compensator neck; the compensator element has a conical compensator shoulder that joins the compensator neck and extends in a direction away from the first end of the compensator element at a diverging angle; the cap has a cylindrical cap neck that circumscribes the compensator neck; and the skirt joins the cap neck and extends conically around the compensator shoulder and away from the first end of the compensator element at the diverging angle. a rotary pump; an electrical motor operatively coupled to the pump for driving the pump, the motor being filled with a motor lubricant; a seal section located between the motor and the pump and comprising: an inlet port to admit well fluid in the seal section, and a communication passage that admits the lubricant from the motor into the seal section; a flexible compensator element located in the seal section and separating the lubricant from well fluid entering the seal section; a drive shaft for rotation by the motor extending through the compensator element; and a cap mounted around a first end of the compensator element, the cap having a skirt extending radially outward relative to an axis of the shaft to limit expansion of the compensator element in a first direction; characterised in that the first end of the compensator element comprises a cylindrical compensator neck; the compensator element has a conical compensator shoulder that joins the compensator neck and extends in a direction away from the first end of the compensator element at a diverging angle; the cap has a cylindrical cap neck that circumscribes the compensator neck; and the skirt joins the cap neck and extends conically around the compensator shoulder and away from the first end of the compensator element at the diverging angle. an inlet port to admit well fluid in the seal section, and a communication passage that admits the lubricant from the motor into the seal section; a flexible compensator element located in the seal section and separating the lubricant from well fluid entering the seal section; a drive shaft for rotation by the motor extending through the compensator element; and a cap mounted around a first end of the compensator element, the cap having a skirt extending radially outward relative to an axis of the shaft to limit expansion of the compensator element in a first direction; characterised in that the first end of the compensator element comprises a cylindrical compensator neck; the compensator element has a conical compensator shoulder that joins the compensator neck and extends in a direction away from the first end of the compensator element at a diverging angle; the cap has a cylindrical cap neck that circumscribes the compensator neck; and the skirt joins the cap neck and extends conically around the compensator shoulder and away from the first end of the compensator element at the diverging angle.