Guys, this is a very old problem, but didn't not show up in a search, so here it is again. What is the cause of my brake light warning coming up when I use the breaks? It does not come on every time, but most times and I need a quick answer without going to the manual or other forums/subjects. Thank you for any answers!

What vehicle? Have you checked the fluid level?

69 roadrunner, and I just checked the fluid level, that was a little low, so I filled it up to the top; we'll see what happens tomorrow, I need to drive the car about 30 miles. By the way, the power breaks work too much, I.e. they tend to lock instantly, which is very annoying and embarrassing when in traffic! I's time to put a proverbial egg under the pedal.

when one side of the 67 & up brake dual system has a leak/loss of psi it makes the electrical valve in the prop/combination shift over to the low psi side completeing the circuit & letting the driver know that exactly that has happened & that the system needs attention before an accident happens. when the system is fixed/sealed back up you need to crack a line on the low side & bump the brake pedal till the valve shifts back over to center which breaks contact & the light goes out. if the system is for sure sealed back up after a prior repair then you are safe but that light is bothersome (which is why it is there!). On the touchy pedal I would unbolt the MC & pull it forward a bit away from the booster so you can check the clearance from the pushrod to the bottom of the rear piston "thimble" (want .020"). If that is OK then it points to a check valve or booster itself issue. EDIT meant to say crack a line on the side opposite the low psi side

Was the booster changed recently?

Might also verify that the park brake switch is not mis-adjusted