Brake Lights staying permanently On
The brake switch on the early production versions of the MGF was designed to be self-adjusting.
Rover modified the brake light switch in May 1998, and replaced it with a different design. They issued a Technical Bulletin No. 0008 on the 6th May 1998 covering all MGF vehicles up to Vin 036126, details of which can be downloaded below.
The problem centres on the fact that the original design was meant to be self adjusting, but "unnatural upward movement of the brake pedal causes the brake switch self adjuster to over adjust". I discovered this after a recent emergency stop (not quite successful a stop, as I hit a tow bar on the vehicle in front! ). The picture below shows the schematic principle of the original design.
The switch consists of an external plunger (black in diagram) that fits and is held inside a secondary internal plunger (green) by a notch locating in a series of grooves. When the brake pedal is applied the spring pushes both plungers in the direction of the red arrow, releasing the pressure on the contacts and allowing the circuit to light the brake lights. When the pedal is released it pushes both plungers into the housing thus breaking the contact.
The two outer halves of the switch clip together, and can be gently prised apart, but you probably will not need to do this.
If the pedal is lifted and moves further up from the normal rest position then external plunger slides further into the internal plunger, and the notch locates on the next groove. When the pedal is returned to normal position this leaves the lights on because the internal plunger no longer breaks the contacts.
The switch is found up against the bulkhead next to the brake pedal itself. To remove it, you must rotate the switch body about 90 degress, until the slots line up, and then pull it down out of the bracket.
The remedy is to remove the switch and forcibly pull the plunger back out. Then replace the switch. Don't allow the pedal to fly back up to its rest position, as would happen if your foot slipped of the pedal when you were braking. This will cause the problem to happen again because the pedal overshoots and pushes the plunger in too far.