The Stovebolt Site Forums The Parking Lot Big Bolts brake light pressure switch question:

It's the 2ton 57 GMC LCF Every thing new in the brake system. Had to pull one of the rear hubs to stop a line leak. Now after bleeding good, still got great brakes, but the brake lights stay on real-real dim.
It has a new brake light switch that is installed in a tee in the brake line. Was working fine before.
I do seem to have the right rear brake drum adjusted up a little too tight.
I can unplug one of the wires to the switch and the lights go out.
1. Would the adjustment cause this?
2. Has my new switch gone bad?

Air at switch.
Bad switch.
Loosen brake at rear to see if it does anything.
I never use those. I always use the lever switch.

Either bad switch...
Or if the MC has a residual pressure valve built in (common), or if one was added, there could be a few PSI held in the system and holding the switch on. Perhaps that model switch is an extra low pressure to operate and one a few PSI higher will resolve the issue.
Another possibility is the pedal is not adjusted properly and needs more free travel at the top, let the MC fully return and relieve pressure.

One test for bad switch or these other possibilities is with the light on dim as is the problem crack one brake bleeder screw to relieve pressure if any. If light goes off then switch is probably not bad but may yet be to low of a pressure to operate correctly in the system.

Thanks, will try backing off adjustment first,
Evidently this truck did not come with a lever switch. I have not found any wiring or signs of where one was mounted.
The original wires ran back to a pressure switch, out before the hydrovac.

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...

Someone with a similar truck needs to advise if they had lever switches or not. At any rate, a lever switch may work somewhere mechanical. Hot wire for brakes into switch, hot wire out and back to lamps.

Will experiment this week end, I am only using two - 4 inch round LED lights for stop lights.

That right there is very likely the problem in itself
They take so little current to light that your switch can have just enough of a current path even when it ought to be open.

just for fun put a (real not LED) test light from ground to the brake light wire and see if that is enough load to keep the brake lights off.
If so you can still try replacing the switch but it may happen again.