Brake light stay on after master cylinder replacement


Just completed a rebuild of the front suspension on our 67. Replaced everything but the coil springs. Replaced the master cylinder , bled the brakes and now the brake lights are stuck on when battery is hooked up. Replaced the brake light switch, still same issue…any ideas? Could we have knocked something loose replacing the master cylinder? Very frustrating… :cry:

Do you mean the brake lights (at rear of vehicle) or the “BRAKES” warning light (on dash, left of speedometer)?

Mine did the same thing until we ended up putting a spring on the pedal to pull the pedal up all the way.

Unplug the electrical connector on the brake switch and check the switch with a multimeter to ensure it is the switch staying on. Pull up on the pedal and see if the switch opens the contacts. If it does, is there a way to adjust the brake booster pedal rod to raise the pedal that way?

Yes, the rear brake lights. I am thinking it is the master cylinder rod…we measured the thickness of the rod where it goes into the switch and it is
thicker on the new one…possible making the switcxh ground out? Haveanother one coming this afternoon

Thank you all for the replies, I will check all the suggestions and let you know the outcome!

Success! It was the master cylinder rod! Took the old one out and put in the new master cylinder and all is well! I guess it being thicker it was shorting out the brake light switch? Either way, its fixed now! Thanks for all the replies!

Im having the same problem. I replaced my Master cylinder and a new brake light switch. Now my rear brake lights stay on when connecting the battery. I connected a volt meter to the switch terminals and they are “Hot” when the pedal is at rest. Brakes are bled properly, all washers and plastic collar on booster rod are in place. Do i need to adjust my master cyl. push rod length to get more play in it to turn switch off? I dont understand what you are talking about when you mention “thickness” and “shorting” it out…

Do i need to adjust my master cyl. push rod length to get more play in it to turn switch off?

Yes, need to adjust the rod length until the switch goes off with the pedal at rest.