Have you got 12 V ish at the bulb holder with the pedal pressed?
Frequently issues like this are down to a poor earth which often shows up at the connecting plug for the cluster as a corroded earth pin and other bulbs in the cluster lighting faintly when they shouldn't - the so called disco lights effect.
I don't know if that car has a single or twin relays for the brakes. If it's a twin set up then swapping over would eliminate them if the fault stays put.
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