[Penthode]

As this thread has gone cold without any resolution and since my methodical approach has gone unnoticed, I thought a picture or two would take the place of a 1000 words.

There are clues in the original post which leads to the direction this should take. As a professional electrical engineer for nearly 50 years, I can point to the problem and it's resolution which should be quite simple to resolve if you follow the clues and apply a logical resolution process.

So let's first start with three clues.

1) the lights dim when the brake is applied.

2) Even when the brake is not applied yet the headlights are on, the horn gives a feeble "eeek".

3) The generator appears to provide a good charge.

Initial Conclusion:

1) The problem is not unique to the brake circuit because the same problem persists with the lights and horn only applied.

2) The generator appears to apply the correct charge. Also the battery is relatively new and does not appear to have been abused. There is no mention of any voltage or currents and so we can guess that the generator and battery are probably okay.

3) The problem therefore appears to be excessive electrical resistance in the path from the battery to the lights and horn.

The attached diagram with the wiring marked in red is the path from the battery to the common distribution point to the lights and horn. A high dc resistance anywhere along this path will yield the problem described. Because of the very low supply voltage involved (6 Volts) and the relatively high currents (4 amperes plus), the wire size must be of ample gauge to handle the current plus the connections must be excellent.

As I suggested earlier is to obtain a digital voltmeter and with the circuits switched on to demonstrates the fault, a table of voltage measurements can be made.

First put the meter across the battery terminals directly with everything switched off and take and record the reading. Then switch on the circuits which demonstrate the problem and measure the battery again. The voltage drop should be zero or very small indicating the battery is good.

Next, with the circuits switched on, measure directly from the positive battery terminal to the chassis. The voltage should be very small or near zero. If you measure 0.6 volts or more, then you have a battery connection problem at this point.

If the reading is near zero, with the circuits still on, measure the dc voltage at the cut out terminal to chassis. Compare the reading with the battery: if it is less than the battery by 0.5 volts or more, then there is a problem with the wiring between the battery negative terminal and the cutout connection including the ammeter resistance.

If the voltage at the cutout matches or is close to the battery voltage, then the problem lies down stream. A major culprit is the connection to the terminal I highlighted in yellow on the headlamp switch. Make sure the lugs are tight. The newer switched use slide on connections which are crimped. I had a problem here which I resolved by soldering the crimped connections.

Also dim incandescent 6 volt lamps is a symptom of using too small gauge wiring in the circuit. The wiring for 6 volt cars must be noticeable heavier in 12 volt cars. For the headlamps, I would even recommend 12 gauge wire from lamp switch to the bulbs and 10 gauge upstream. Anything less will raise high resistance and will lead to dim lamps.

To finish, the symptoms described point to a wiring problem between the battery and light switch.