Changing to LEDS can prevent the old-type flasher unit from flashing correctly, since LEDs use less power (watts) than a normal lamp. You can buy special hi-load LEDs, or use an expensive load-independent flasher unit.

But other than that, LEDs should cause no problems, and using an LED lamp for the BRAKE warning light is OK

If the BRAKE lamp glows dimly all the time, it means some circuit is shorting to ground. The BRAKE lamp is HOT (+12V) all the time IGN=ON. It should only glow when the handbrake switch connects it to ground (earth) or when the master cylinder cap sensors connect it to ground. If you disconnected those, and it still glows, the short is somewhere else

The colors shown here are for B110, for the later B120/B122 the wire colors are somewhat different