1997 GMC Sonoma SLS 4 Cyl 2.20L
Brake light goes out if press brake hard
When brake is pressed softly you can hear the first of three contact in brake switch make and brake lights come on. If you push harder you will hear another set of contacts click and brake lights go out. So anytime you press hard on the brake, the brake lights will flash once and go out.
At the moment, I suspect your brake switch. If the position of the brake switch is adjustable, I guess you could try adjusting?