I bought a 1997 f150 and noticed that the brake master cylinder has an electrical connection that was disconnected and taped up. What is the switch used for on the master cyclinder? Is this a sign of a bad master cylinder that was not replaced?

 

You talking about the lil connection on the bottom of the master cyl.
Its a magnetic switch , if you look in the brake rev. you should see a lil white disc in there ,

When it gets low on fluid , the disc drops to the bottom of the rev. an trips a lite to tell you that your low on brake fluid.{ pretty cool idea }

Sometimes it gets a bad ground an will set off the lite ,

 

If it's the connector at the front of the master cylinder, attached to the aluminum, it's the Brake on/off switch. It's used by the cruise control to let it know to disengage when you step on the brake. If it's disconnected, your cruise control should not be working.

Alot of Ford trucks built from 94-02 are under a temporary fix recall to replace either that switch and the harness or just the harness itself. The switch can start leaking brake fluid and lead to a short and fire in some rare cases. If that switch is disconnected and taped up, you've had the "interim repair" performed. You should get a notice in the mail when the harness become available. Late November(waiting) early December.

 

Thank you for the reply that is why the plug has been removed. I didn't take the truck to the dealership and had not looked under the hood since this had been temporally disconnected.