Ok, so that big "compression chamber" is actually a vacuum operated booster for the hydraulic brake system, and that's where the stoplight switch is located. The other "chamber" you see on the rear axle is a vacuum operated shifter for the two speed rear axle. This is remotely operated by the **** on the dashboard.
Hope this helps

 

Pic on left shows the master cylinder with the stoplight switch. Pic on the right is an older GM OEM electric fuel pump.

 

Thank you so much for helping me identify them. So it is a fuel pump, I guess it is grounded on the frame since there is only one wire.

When you say stop light switch? I see that reference in the shop manual, what is the function of this?

I have another topic post asking about the brake hi/lo switch that I think goes with this discussion.

https://www.ford-trucks.com/forums/1...l#post15655489

 

The stoplight switch is a normally open (NO) circuit pressure switch that closes the circuit with a rise in pressure. Basically what that translates to is the stoplights light up when you press on the brake pedal.

 

Good to know, I thought maybe it ('stoplights') meant more than that. Thanx.