A vehicle heat pump system includes a compressor, a first valve coupled to an outlet of the compressor, a cooling circuit between the first valve and the inlet of the compressor, a heating circuit between the first valve and the inlet of the compressor and a controller. The heating circuit includes a heating circuit evaporator and a second valve between the heating circuit evaporator and the inlet of the compressor. The controller is configured to operate the first valve to switch between a cooling mode and a heating mode, cycle the second valve opened and closed in the heating mode, such that when closed, the compressor draws refrigerant out of the cooling circuit and refrigerant pressure builds up within the heating circuit, and maintain the second valve open upon the controller determining that sufficient refrigerant has been drawn out of the cooling circuit during the cycling of the second valve.