…why the Monotype manuals say not to raise or lower the pot unless it is molten. They never explained why this could cause any problems, and the pump piston linkage is jointed enough to follow the pot up and down with no trouble, so I never gave this much heed.
It turns out not to be the pump piston linkage that is the problem, but the pump lowering lever. If the metal hardens when the pot is not fully up in its operating position, the pump will be frozen in the metal in a raised position. If you then crank the pot up to operating position without fusing the metal first, the lever that lowers the pump away from the mould will break.
Fortunately I have some spares of this lever. Also fortunately, this lever breaks rather than the ones that actually support the pump body; there are two of these and they are much more trouble to replace.
Because the pot is so heavy, the crank to raise it has a large mechanical advantage, so the force required to break this lever isn’t even noticeable.
Although this happened to me with a pot that solidified in a lowered position, it is also possible for this to happen if you raise a pot the had hardened in the raised position and has since been lowered. This might occur if, for instance, you have adjusted or replaced parts of the pump body lowering linkage.
I’ve learned my lesson, and now that I know why raising a cold pot is a problem, I should remember never to do it. It still seems, though, that lowering a cold pot is OK.