One brake light/tail light is brighter then the other????

 

Check the sockets for continuity using a multimeter.

Let me guess, is the driver side the brighter of the two? Whenever tail lights have issues, usually it's the passenger side first. That's because the wiring to the passenger side is longer and has slightly more resistance than the wiring out to the driver side. If both sockets age at the same rate, the passenger side will see issues first, usually due to ground shift.

If I guessed wrong, then nevermind (but still check the sockets). Use a multimeter - there's no way to "see" resistance. You can't use a ruler to tell how something tastes and you can't use a thermometer to tell how loud something is. Electricity is no different - use the right tool for the job.

 

Mine are the exact same way.... but my brake light will go completely out when I hit the brakes if the headlights are on.

works fine if headlights are off.

I was told to check all grounds and then buy a headlight switch

 

The difference in brightness can also be as simple as dirt/dust inside the light. It's amazing what can be accomplished by pulling the light units and washing them out.
But, a ground issue, or corrosion in any of the connections going to the rear of the truck are always things to look for, even before you start having lighting issues.

 

I have a 1979 F-100 Custom and my truck tail lights don’t come on unless I step on the brake. What do you guys think could be causing this?

 

The rear bulbs have two filaments - one for the parking (aka running aka tail) lamps, the other filament is used by the brake lights/turn signals/emergency flashers.

You need to be sure you're looking at the correct problem.