1976 K100 Cabover project

The valve should look like this or similar if a different brand. It should have the hoses for the trailer brakes connected to it unless they removed them and / or plugged the ports. The red knob valve has a hose that connects to this valve also to supply air to trailer to release its park brakes. Back then the yellow knob normally only controlled tractor park brakes and was independent of the red.
The blue knob should be connected to a reserve tank that is isolated from the rest of brake system somewhere usually under cab. All it does is give you an extra supply of air to release park brakes in an emergency if all the other system air is lost. Another California mandate originally. We used to call them California Bus Valves.
The systems back than were much simpler and a bit different than todays. Less parts and b.s. and accomplished the same thing. It
Stopped the truck.
BTW. Does she have steer axle brakes?

 

If you are gonna use the truck as a straight truck and never hook an air braked trailer you do not need a protection valve at all. I would remove the lines entirely and plug the red valve on the dash and if you wanna keep the hand valve for looks plug it too. If not you can remove it completely too. Its only needed for trailer control. That way if red valve is pushed or hand valve is pulled no air will be lost. And that way you or the next caretaker can always plumb it back up for a trailer if need be.
Reason I asked about steer axle brakes is they were not required by law until mid 1980. So most three axle trucks up to ‘80 I’ve seen or owned never have them. But you said it has power steering so most likely thats why. Majority of manual steers did not.

 

That air line is gonna be connected to the foot valve or a check valve under cab. Trace it back and disconnect and plug. Its really not safe to leave it there or cap the end since its part of the brake system. If it ever ruptures you will have no brakes when you hit peddle. Best thing to do for an aux. air port for air tools or what not is to put a good quality quick disconnect like you have on end of airtool hose into a non used fitting in the air tank.
That vintage Kw and Pete did not use fuses they used circuit breakers. I cannot remember where they are located on The Kw’s but on the Pete’s they were under the dash behind the light switches above the dog house. The table like top of dash is hinged and flips up for access. Remove a couple screws.
My godfather had a ‘77 K100 from new for about 20 years. Unfortunately he is no longer with us to ask and the truck has been MIA since he sold it.

 

Forgot to mention. Check the smaller line by pushing in the red valve and see if air comes out of it. If so plug it so if the valve is ever accidently pushed you won’t lose air. And just to be safe make sure it doesn't do it with the yellow or blue also.