Abstract: A gravel packing tool includes a differential piston sleeve normally covering an auxiliary return port through the wash pipe located above an annular seal of a sand screen liner. The seal engages the wash pipe sealing against pressure communication between upper and lower sand screen sections in the liner. A shear pin connecting between the sleeve and the wash pipe holds the sleeve in its port closing position until the pressure differential between the inside and outside of the wash pipe causes the sleeve to shift into a second position breaking the pin and exposing the port to establish an auxiliary path for liquid to return through the wash pipe. A spring acting between the sleeve and the wash pipe urges the sleeve back into its closing position when the pressures inside and outside the wash pipe begin to equalize.