Step one: check bulbs (you may have blown stop on both. Unlikely though)
Step 2: check switch above pedal, maybe it's sticky and not moving when pedal is depressed.
Step 3: report back.
The tail/brake light wiring assemblies are prone to developing faulty earth connections also on these cars. It causes some weird effects like the tailights actually dimming when the brakes are applied. Can be corrected by running a supplementary earth wire to the body if that's the fault.