1992 Plymouth Voyager ABS brake fluid pump

I have a 1992 Plymouth Grand Voyager with ABS and 3.3L v6 engine. The electric power brake pump activates for a few seconds, then stops, then about 5 seconds later it activates again, and again and never stops cycling. There is no fluid loss and the brakes still work. It acts as if the fluid is passing back through an internal check valve and bleeding off the pressure. Do you suspect the problem is in the pump unit that is mounted on the transmission or the master cylinder unit above? Thanks so much. Any help or references appreciated. Thanks, Gene