OK. I have a '49 and the brake lights will randomly come on and stay on while the vehicle is parked and drain the battery and while driving. I can park and the lights will be off and then at some point in time the lights will just come on and stay on. I have replaced the brake light switch, master cylinder, wheel cylinders, brake lights and housing, and rewired everything from the switch back. Any ideas on what could be causing this and how to resolve it?

 

I had this problem, I had to adjust the brake pedal. It was out of adjustment just enough to apply enough pressure to the switch to activate the lights.

 

The brake lights are a fairly simple circuit. Basically there is power supplied to the brake light switch at the master cylinder. When you step on the brakes the hydraulic pressure activates the switch connecting the power with the rest of the circuit.

If the brake lights are coming on by themselves then somewhere the circuit is shorting to a power source. The most likely place for this to happen is in the switch or at or near the connector to the switch.

Is it the brake lights coming on and staying on? or is is the tailights? Does your truck have one of those accessory turn signal levers on it? if so then the brake light wiring probably runs up and through that switch and that switch could be a source of the problem.

Bobby

 

no turn signal switch and no shorts. It had been doing this for a while and that is what prompted me to rewire and replace all the electrical and switches for the tail/brake lights. Come to think of it, it may have started doing this after replacing the master cylinder. Any connection there? Could the master cylinder be building up and applying pressure on its own?

 

It shouldn't. Maybe the weight of the brake pedal is enough, and the switch is really sensitive it might be able to happen.

You might take a look at how much brake pedal travel you have before the lights come on and see if that tells you anything.

Bobby

 

If you didn't adjust the brake pedal after you installed you new MC that could be your problem. Your old MC was worn and might have allowed the pedal to fade, when you installed the new unit the pedal is pushing into the MC causing slight extra pressure on the switch causing it to engage. Mine did the same thing, the lights didn't go on until I applied the brakes and then the switch stuck on. I didn't realize what was happening because I parked the truck during the day and didn't notice the lights were on. I'd come out and the battery was dead. I'd jump start the truck and everything worked fine but it would be dead again the next day when I went to start it. I just thought I had a bad battery and was planning on getting a new one. Then one day I was going some place and my wife was following me. I stopped at a gas station and when I took off my wife honked her horn to have me pull over. She informed me when I left the pump my brake lights wouldn't turn off. I was able to crawl under the truck and tweak the pedal adjustment and haven't had the problem since.

I would assume one the truck was parked for a while pressure was alleviated and the lights would turn off but as soon as some pressure was applied the MC would retain just enough pressure that the switch would activate.

 

Bobby, I think I said that in my first post

Just want to give you a hard time

 

Sounds like you nailed it Bob, I'd wager a frosty beverage that you're right

Bobby

(yea....in my defense I think we were posting at the same time.....at least thats my excuse and I'm going to stick to it....LOL)

 

Thanks guys. I'll try re-adjusting the pedal and see if that resolves my issue.

 

Bobby, shouldn't you be working instead of posting quick replies?

I'm self employed, I can do whatever I want as long as my wife isn't around

 

Well, lets take Bobs idea one step further. Did you reinstall the brake pedal return spring after your swap out. Could be that it's sitting on the piston and causing th epedal to activate slightly.

Just food for thought.

 

working? working? oh yeah, I guess I'm supposed to be working. Dang, thanks alot Bob, ruin my whole day.....

 

How close is a brake line to your muffler or exhaust manifold?
This was my problem, the exhaust pipe was heating up the brake line to cause enough fluid expansion which pressurized and activated the switch. Then when it cool down the brake lights would go out.

Dale
Ford52F2

 

Readjusted the brake pedal. So far so good no lights on. But I did notice that there are no brake or clutch pedal return springs under there. How and where do the return springs attach?

 

I'd have to look under one of my trucks but I know the brake spring goes from the pivot peg to a hole on either the transmission bellhousing or engine block. It's just a small unnoticeable hole, kind of in a straight line from the brake pedal. Do a search of the forum, I know it's been asked before.