brake wear light?

Brake pad sensor works the same as the piece of metal that sticks out when pads are low enough to need changing, the sensor picks up that the pads material is low and sets off the light on the dash.

New brake pads should come with a new brake pad wear sensor, usually I think its just 1 sensor per axle, so you'll see if its on the drivers or passengers side when removing the wheels.

A new sensor will reset the light on the dash automatically as far as I know.

Take note of what way the sensor is routed and make sure to put the new one in the same way.

The sensor itself at the end goes onto a little cutout on 1 of the pads

Yes but that's a BMW where everything must be reset by electronic means. I believe on most cars that once you put in a new sensor, the light will automatically reset, maybe after a drive cycle.

The only really bad advice here is telling people to force fluid backways through a master cylinder.

What should they do when the seals get inverted?

You will be fine so, just maybe blow any dirt or grit away from where the sensor goes, and you will be fine. All the electrics will be switched off during the job anyway so, no settings disturbed. And unless the disc warning icon was on before you started the job, nothing to worry about, ( and even if it was, the new disc's should take care of that. Good Luck with it.

With every post you make,it becomes more obvious that there are lots of things you havent seen.

You know nothing about master cylinders either if you think that there should be back pressure on them.

Difference between us is that I know - you spoof. I just don't need to impress anyone with my knowledge.

You being master mechanic, It's sad that I have to explain this to you but here goes.

Seals can & have been inverted where fluid is forced back faster than the return valve in the master cylinder can take it (By pushing the caliper piston back too fast).

If you're unfortunate enough for it to happen,the cure is : 1 remove master cyl. 2 throw in rubbish. 3 replace master cyl & so on. Quickly turns a simple job into a big job.

The proper process is use brake bleeding bottle,loosen nipple,push back piston & tighten nipple. No bleeding necessary & the brake fluid reservoir isn't overflowing.

That's first year apprentice stuff,you shouldn't have to consult a manual for it.

“Never argue with an idiot. They will drag you down to their level and beat you with experience.”

― Mark Twain

Taken from the Haynes manual of bad advice