Hi Tale:
So my 76 does not have the little switch that your does but other than that the foot pedal assembly looks the same. So it looks like the GM guys added this feature to notify you of some misadjustment of the parking brake.
Assuming the rest of your parking brake system is the same as mine, you should have a 3 cable arrangement: #1 cable from the brake pedal to the #2 cable; #2 cable to #3 cable; and #3 cable to each rear brake(there are 2 of these cables). Between cable #1 and #2 there is an equalizer nut that allows adjustment...it is underneath the truck on the drivers side near the transmission tailshaft.
Assuming your brakes a working fine, here is the adjustment:
1. Raise the rear of the vehicle so the rear wheels are off the ground...make all the prudent safety precautions as you need to get under the vehicle.
2. With the parking brake off, the rear wheels should rotate freely...if not, you need to adjust your rear brake shoes.
3. If they rotate freely, set the parking brake to the first notch from the fully released position.
4. Tighten the equalizer nut until a moderate drag is felt when each of the rear wheels are rotated forward.
5. Fully release parking brake and rotate the rear wheels...no drag should be present.
If all this does not set things right, maybe you have some corrosion or crud in #1 cable preventing the cable to slide freely in the jacket(conduit). Get some cable lubricant (not WD40) and use a spray tube to spray it down the cable jacket. This would be the same lube that motorcycle and bicycle guys use for their brake cables. You can also get it at bicycle stores.
Regards,
Henry