Pressure switches have always been a problem both on the electrical side and on the hydraulic side. When a switch sees a small pressure and starts to close it arcs. The greater the number of lamps the worse this gets. If the switch sees a pressure just enough to start activation but not enough to fully close the contacts it can be burned up in a very short time by this arcing and the heat it generates. If you are 6V this is even worse because currents are higher for a given total wattage of bulbs
I have seen cases where a weak return spring on the peddle and or in the master cylinder result in enough pressure to start closing the switch. If the push rod into the master is slightly too long in adjustment the relief port won't open and as the brakes heats the pressure will build. This will eventually also cause brake drag.
Everything else being in good shape, I don't think the shoe adjustment is the problem. If adjusted too tight, the hydraulic pressure should still reach zero when the peddle is up. I would not expect residual pressure valve, but if there is one, that could be the source of pressure holding the switch partially closed.
The switch may well be bad now or have been defective from the start. I don't know how many bulbs you are lighting, but if you are switching a lot of current, the switch may not be designed for it. A horn relay may be the solution if the current is too high for switches you can get today. Adding this to the circuit will reduce the switch current by more than ten times.
If you open a bleeder and the switch stays on (or partly on) it needs to be changed.
These switches have also been the cause of mysterious flat batteries...
