The pressure is "boosted" i.e. just higher in the brake line than in the master cylinder because the line is so much smaller in diameter than the cylinder. I understand where putting the switch in the line would help operate the switch, however that might indicate another problem with the brakes since the switch is designed to work at the operating pressure of the Master Cylinder.

If it were purely a pressure problem, the brake lights would never work. My problem is that they work with a new pressure switch for about 2 weeks and then, nothing. Checked the switch with an ohm meter while my wife operates the brakes and it doesn't close. I have the correct voltage to the switch.

It acts like the internal mechanics of the switch are breaking down.

 

Not true, the pressure is exactly the same everywhere in the system.

 

Hmmmm......what was I thinking? I was confusing pressure with volume. Well then, putting the switch in the brake line instead of the master cylinder really makes no sense.

 

Nope, not in this case. It adds two joints that are potential leaks.

 

There is a good reason they no longer use pressure switches for brake lights. I would sugggest installing a mechanical switch that will be activated by the moving of the brake pedal.
Also whether or not the switch is used with 6 volts or 12 volts is not the problem. The switch doesn't care. Adding a relay would only seem to add another potential component to fail. A relay should only be needed if you have so many lights it exceeds the rating of the switch. The standard two or three lamps should not be a problem.

 

Probably true (about using a pedal-activated switch) but there's no obvious way to do that on the stock pedal arrangement. Besides, there are dozens of us with the same truck, same hydraulic switch that have not had any problem whatsoever.

Doug, why don't you hook up a test lead from the battery (+), with a 10 amp (maybe 15) circuit breaker in it, and connect that to the wire heading off to the brake lights. See if it pops the breaker, smokes, or there are any other dramatic results. Leave them on for 5 minutes, see if there's anything getting warm...

By the way, you do have stock brakes, don't you? No booster?

 

C1AZ-13480-A original Ford part number.

 

It's all stock. I replaced the switch for the time being and they seem to be working ok. I wonder if it wasn't just a bad run of Chinese switches.