Hello, I hope the following assists you.
This will be due to the stop lamp switch either moving through been loose or being out of adjustment. When you press the clutch the pedal box that the clutch pedal pivots on flexes, the brake pedal also pivots on this pedal box so if the stop lamp switch is loose as the pedal box flexes the contact is made inside the stop lamp switch.
Look up near the brake pedal and you will see the switch it has an electrical connector to it, the switch turns about half a turn to remove it from the pedal box.
I hope this helps but if you have more questions regarding this let me know and I'll reply as soon as I'm able to.