Brake lights not working 1979 Dump Truck

[Gunny65]

I am picking up a 1979 Mach Dump Truck and the brake lights do not work.  The turn signals work, reverse lights work and running lights work but no brake lights (both sides do not work). I am going to pick up the truck tomorrow and need to figure out how to get them working before I leave the yard.

Do these trucks use glass tube fuzes?

Where is the fuze box located?

Is there a "switch" on the brake pedal that could be bad?

Do they come on when the air pressure is high enough or does the pressure even matter?

Any suggestions on what to look for first or how to do an emergency bypass just to get it home would be appreciated.

I have a fluke meter and a "basic" understanding of how to use it.

Attached is the only picture of the truck I have at this time.

Thanks.

Edited October 11, 2023 by Gunny65

[Geoff Weeks]

Yes there has to be air pressure, but not much 10 psi should be more than enough to make the switch activate.

[mechohaulic]

I would check power to and from brake light switch 

[Gunny65]

Not sure where the brake light switch is located. It isn’t in the cab. Pictures later that might show it but not sure. 

[Joey Mack]

I wonder if it is on the firewall, in the center behind the valve cover, like it is on later models ?

[JoeH]

There's 2 brake light switches. One comes on when air pressure is sensed in the brake lines. The other is set to kick on below a certain pressure, when air in the parking brake circuit is dumped. If I recall, the service brake sensor should be attached to an airline on the interior firewall above the brake pedal. The parking brake one is probably on the parking brake knob in the dash. It might also run the low air pressure buzzer and light.

[Glensts]

On my 79 there was an air pressure switch under the hood attached to the brake treadle valve.  Just 2 wires and one had constant power and one went to the lights.  

[doubleclutchinweasel]

My '70 had the parking brake switch beside the parking brake valve and the service brake switch at the treadle valve.

[Gunny65]

I did find the switch and it is bad.  It is right under the floor board, outside of the cab, where the brake pedal air hoses connect. When I hooked the wiring back up, it just made the brake lights come on at all times.

Thanks all.

Edited December 2, 2023 by Gunny65