Abstract: A wafer processing apparatus is provided with sealing ports between adjacent evacuatable chambers that are actuated to compress elastomeric seals carried by the valve sealing elements to differing degrees of compression, based on the amount of pressure differential between the chambers when sealed. The degree of compression is controlled so that less compression of the seal takes place when less is required to seal, such as with lower pressure differentials, thereby avoiding unnecessary fatigue and wear of and around the seals that would otherwise increase the generation of particulate contamination.