Sometimes the horizontal grids break loose on the evaporator plate. This causes the ice slab to hang on. There is no repair, however you can use a pair of pliers to pull the loose grid completely off. The machine will still make ice, just with a few extra large cubes. B model...at least ten years old...time for a new one.
I'd tend to agree about not hooking up the gauges until later. I went to a Manitwoc school and the rep insisted the first thing to do is whip out the book, that was a few years back, and as it turns out he is right.
Self contained (no remote condenser) systems are critical charge. If system is acessed on every service the unit will indeed someday be low on charge. If you're the last dude in line and find it low now we are looking for a non-existing leak.
Better yet, buy a low and a high gauge get a couple of fittings , 1/8" female pipe to 1/4" sae female. Make a set of handy dandy gauges. One for each shirt pocket. No hose...miniscule refrigerant loss.