Brake light switch 72 dart

so adding the new booster set up has lowered your peddle?
in whch case have you got the right pushrod length?
and depending on year is it in the correct hole in the brake peddle lever. The pivot point was different i think between manual and assisted brakes.
if you do have the right rod in the correct pivot point, adjusting the switch position may be necessary

However if it only does this when the motor is running or when the motor is running and after pressing the brakes. the booster is still slightly on and the brake lights will go off and on depending on vacuum/throttle position/load.
that issue needs to be adressed first before you set your switch.

why do you mention new booster and still drums? is the master cylinder a drum master cylinder or a disk/drum master. a disk/drum master will be missing the residual pressure valves under the place where the brake pipes attach for the front brakes
that might mean the peddle droops becasue the pressure in the front circuit is not enough to bring the master cylinder back to "rest".

you would normally pull them out with a wood screw if using a drum master with disks or (modern car drum cylinders like off a dakota truck) although thats not great becaue you need the big and small resevior master cylinder for disks/drums they come with a sensible bore for the mixed application.

Dave