Back Flow On Guzzler Hand Pump.

Your pressure is too high. Clean your filter press to lower the pressure. The reason you are experiencing back flow is because the pressure inverted the inlet check valve flapper. The flapper can be dislodged to fix this. (Look into the pump’s inlet to see the lodged flapper). There is also an umbrella valve option. This option became a standard, factory installed part on all presses released after 5/31/2018.