Shower pump keeps running when taps are turned off?

My friends have a pump to boost the pressure to both hot and cold feeds to their bath and shower. The union on the hot water inlet pipe developed a leak, so a replacement pipe was fitted but now the pump carries on running after the taps have been turned off. We've tried "purging" the system by running the tap with the pump switched off but to no avail. The pump speed increases when a tap is opened and reduces when the tap is turned off but doesn't switch off completely. Any suggestions gratefully accepted! Thanks, Bob.