For some vacuum applications it makes sense to install a discharge recirculation line to destroy the vacuum in the stuffing box area. This becomes very important if you have to seal a liquid that increases its viscosity with agitation. Connect the bottom of the stuffing box to the suction side of the pump to allow a single pass of the liquid through the stuffing box. Make sure the connection is very close to the seal faces. You will be better off using the seal gland flush connection rather than the stuffing box lantern ring connection.
Some liquids decrease their viscosity with agitation.
If this problem exists use one of the environmental controls mentioned above. Flush with a clean liquid. Check with your seal manufacture for the minimum amount of flush that is needed. At shaft speeds below 2900 rpm. filling the seal cavity with a compatible grease is usually satisfactory. Increase the seal clearance in the stuffing box area.