A dump valve having a valve seat and a valve member is located in a string of drill pipe above a downhole motor. The dump valve connects the inside of the drill string with the annulus and allows drilling mud to flow freely therebetween when drilling mud is not being pumped through the drill string to operate the downhole motor. The valve member is made of a deformable resilient material such as rubber. It is held away from the valve seat by spacers until the pressure drop across the valve member is sufficient to deform the valve member and move it past the spacers into engagement with the valve seat. When the pressure differential is relieved, the valve member will return to its normal, unstressed shape opening the valve.