Brake lights don't work when my headlights are turned on

[mduster]

HI Everyone,
I am new to the site and just purchased my first 1978 and I am trying to troubleshoot my breaklights...


What have I done so far:
Replaced the bulbs and clean the connections

Problem: When the headlights are turned on the tailights work but when I hit the breaks the break light doesn't come on fully making it hard for the people behind me to realize I have hit the breaks It happens on both the left and right rear break lights.

It's weird the second fillament lights up but its not bright enough ...

Any ideas is it ground issue?
thanks in advance

 

[Fast Eddy]

It's a pretty tough debugging problem. Get a good digital volt meter and measure the resistances to ground and the voltage coming from the brake circuit. How are the blinkers?

 

[mduster]

The blinkers and side taillights work great...

Since the second fillament lights up it appears to me its getting power just not enough.

 

[Fast Eddy]

Ground is probably ok too. Check the resistance of the switch at the pedal.

 

[mduster]

Sorry for the dumb question but wondering .....

On the stoplight bulbs there are two filaments one on Top which is really bright and the one on the bottom which is less . My problem when the headlights are on the one on the bottom lights but is not as bright.

My Questions: can I flip the wires somehow so the bottom one lights up as my tailights and when I hit the brakes the top one lights up?


I will check the brake pedal switch as you recommended too.

 

[bj40green]

The light bulb should be 12V 21/5 (Europe) or 12V 32/4 (Costa Rica) or 23/8 (USA) depending on the country regulations.
In this example the "brake" filament is the one below and is 32Watts
The "run" filament is the one on top and is 4 Watts

Some people manage to fit the bulb 180° rotated in the socket. Shouldn't be possible because of the two un-equal studs but it happens.
Some people manage to make the wrong connection in the wiring. They mix up the "run" wire with the "brake" wire.
Check this first.

Rudi

 

[mduster]

Thank you I will check out the bulb ! Really appreciate your help