A monitoring system provides a method of operating a compression plastic molding apparatus which entails substantially continuously monitoring the position of an upper compression cam of the apparatus, against which a rotatably movable mold assembly reacts during operation of the apparatus. By monitoring the position of the compression cam, the system is able to detect excessive loading of the apparatus, such as attendant to depositing of excessive plastic material in the mold assembly. A force determining device, illustrated in the form of a fluid pressure (pneumatic) cylinder, is operatively connected with the compression cam, with the system operating to release pressure in the cylinder in the event that the compression cam is moved in a manner which indicates excessive loading of the plastic mold assembly.