I inherited a black and decker plunge router from my grandfather. From when I first tried it the bit has always slowly receded back decreasing the amount the bit protrudes from the router base. This has happened when using it freehand and in a upside down table configuration. The bit collet itself is gripping well enough onto the bit, the bit itself is not slipping down into the collet. No matter how tightly I tighten the height lock mechanism this always happens. I have even removed the plastic cam toggle handle as I read somewhere so I can tighten the lock nut itself even tighter with a spanner, this also does nothing. The lock mechanism works with a bolt which basically seems to press against the rails/poles of the router to stop it from moving up or down. I loosened and flipped the entire base around so that the bolt would be pressing on "virgin" surface, because where the locking bolt repeatedly pressed on the original rail/pole it started to scratch a path into the surface. Doing this also did nothing. The height decreasing/slip only seems to happen once I begin routing. I've wondered if removing the springs could be a makeshift fix? I have no idea what could be the problem. Is the locking mechanism bolt too smooth from being slightly filed smooth over the years? I can't do anything with the router because the bit height always decreases making it almost impossible and aggravating to do anything. Thanks!