This problem has me stumped for the past week, and a lot of research into this issue, both online and in auto parts stores, has left me frustrated. First off, I own a 1995 GMC Sierra 5.0L V8. What I am having trouble with is that my brake lights do not come on, all three. After a few minor checks (bulbs, etc), I could not resolve the issue so I performed a ground up inspection of the electrical system.

Bulbs are intact.
Grounds are good.
Fuses, both the 10A and 30A, are good.
Relay is working.

At this point I discovered the brake light switch issues through some research. Fiddling around, I was able to manually activate the brake light switch by pulling it against the hinge that threads through the switch body (where the brake pedal arm and push rod to the brake booster connect). This causes all three brake lights to activate. This confirms that the wiring and components are good and that the problem with my brakes is at the switch.

Here's where I get frustrated. I replaced the clip that retains the switch onto the hinge, as it was broken, and pressed my brake pedal. No dice. Yet, if I manually pull on the switch by reaching up under the dash and pulling it aft, it functions. After some fiddeling, I assumed that some piece must have broken off and gotten lost. I went to my local autoparts store and purchased a new switch and threw it on. Same problem in that the switch does not activate upon peddle press yet I can reach up by the peddle, pull it, and it triggers the lights to come on.

I am incredibly frustrated because, after some minutes studying this switch, I don't understand how it was supposed to work to begin with. I get that you have three internal contact pairs (brake lights, cruise control, and transmission I believe), and the plunger inside the switch body simultaenously makes these open or closed. The switch body slides on to the hinge along side the push rod for the brake booster then clips to the push rod itself with the retaining clip attached next. HOWEVER, I don't understand how pressing the brake pedal would cause the plunger to be pushed, as there is no relative movement between the pedal, push rod and switch: they are all mounted together and thus the pedal/push-rod hinge cannot push against the switch contact without just moving the entire switch body along with it. Additionally, there is no piece that would cause the switch to be pressed into the hinge, which is how the plastic plunger to activate the switch seems to be designed to work.

All how-to guides and videos for replacing this switch only say pop-off old/pop-on new and you're good to go, so I am at a huge loss here. Does anyone have any insight? Am I missing some kind of piece? I am desperate!

 

I did a youtube search for " 97 GMC Sierra Brake Light Switch " and got 3 results
They are pretty bad videos but informative. The one thing that confuses me is you say you are able to reach under the dash and PULL the switch and manually turn the brake lights on (scratch head) shouldn't the switch work by pushing the switch forward to manually activating the brake lights. No matter. I hope one of those Youtube videos can give you some help.
The one thing those videos shared was the brake peddle would rock left to right so much so that the brake peddle arm would break away from the brake light switch

 

My description on manually triggering the switch was correct, but perhaps I should describe better what my terms I'm using are.

brake pedal - The rubber/metal portion you push with your foot
brake pedal arm - The arm that supports the pedal and connects up underneath the dash
push rod - the metal rod that connects between the pedal arm and the brake booster on the other side of the firewall
hinge - the piece that allows the push rod to hinge on the pedal arm
switch body - the plastic housing of the switch, including all the contact, moving parts, and harness
switch - the part of the switch body which actually moves, physically opening or closing the contacts

So here, the switch body connects by sliding onto the hinge alongside the pushrod, secured with a clip. The switch body also has a clip on its side that snaps onto the push rod itself. The brake pedal arm pushes the push rod when the pedal is depressed, however, the switch body simply moves with the pedal arm and the push rod, so nothing actually presses the switch, which is located inside of the hole that the hinge exists through on the switch body.

By reaching up the pedal arm, I am able to grab ahold of the switch body and pull it forward, causing the switch contact to be smashed against the hinge (in a sense, the inverse as if you were pushing the pedal and thus the arm and thus the hinge [being all one rigid body], except that there is no component from restraining the switch body from simply traveling along with the pedal arm and push rod.

As an analogy, it's as if you're pulling one end of a bungee cord while the other end is unsecured; pulling it does not stretch the cord. Similar to what's happening here, where pushing the brake does not cause the switch to be depressed, because the switch body does not have anything to press against.

I know that this is a hyper-detailed explanation for what a switch does, but that's 10% of my engineering background and 90% of me being fed up with this problem and not understanding how this switch was supposed to be pressed.

 

I have a '96 and a '97 I'll look at tomorrow and take a picture for you. Mine works fine. Mine even has a 5.0L!

 

That'd be great! If you're able to get a picture of the brake switch when the brake is depressed, it might answer my question.

 

I'm still looking to fix this problem. If anyone has any input, it'd be appreciated.

 

Did you figure out the brake switch problem?