Hello, I've recently discovered a problem with my brake lights. When the (1988 Pickup 2wd, power brakes) truck is running the brake lights do not work. All the other lights (Tail, Turn, Reverse, Head, Dash, etc) work, but the brake lights will not work. When I turn the truck off and press the brake pedal once, the lights still don't come on. When I press a second, third, fourth... 20th, time, the brake lights WILL work. I'm not too good at the electrical stuff, but I imagine this means something with the power brakes? This is also reinforced by when I put the key to accessory, they still work. They'll even work shortly after start up (if I've pressed the brake while the truck is off). But after about 5 seconds of the truck being on, they go out again.

I've checked the bulbs and the the fuse (15Amp) as well and they're good.

Any suggestions on what to check?

Thanks!

 

The brake light switch on the brake pedal is sticking or bad?

 

I’ll probably end up taking the switch out to test it but I don’t think that’s the issue because when the truck is off the brake lights work every time the pedal is pressed. When the truck is on, they never work.

 

Well, it's one or the other.
There is nothing in the StopLight circuit other than the Stop Fuse, the lamps, and the switch. Your power brakes, ignition switch, engine computer, differential, ashtray ... have nothing to do with the StopLight. There's a 4-pin connector at the switch

Use a multimeter to check the switch, and if good, the rest of the circuit. Don't be one of those guys with a parts cannon.

 

I've seen tow wiring and/or connections be the root cause of many issues in the tail light.
But follow Scope103's advice. He's very good with troubleshooting electrical.