Rear break lights not working. -Random

Hi. I have to admit that I'm useless with sorting out car issues. So hopefully you can give me a hand.

I have a peugeot 405 - 1991. In the morning when I start my car, I push the break pedals (gently) and all the breaking lights work. When I start to reverse I gently press the breaks again. And NONE of the breaking lights work. I drive forward, use the breaks gently and NO lights. This issue lasts for couple of minutes. After this, everything is normal again. What could be the reasson for this random issue to happen?

I went to see my mechanic and he checked the switch that controls the breaking lights (the one clsoe to the pedal) and lubricated it. Well that is what he told me. Any ideas? Thank you!