A couple of days ago I had the unnerving experience when the brake peddle went too easily to the floor and very little braking effect.
Cause was a failed (corroded) brake line to the right rear. A common problem I understand due to corrosion caused by moisture retained by the foam backing on the top of the undertray in the region of the brake lines.
Brake line replaced and used a power bleeder to bleed the brakes.
A liter of fluid through each caliper but the brake pedal still wasn't right, far too soft, there must still be air in the system.
Found a thread on the Forum which said to bleed the master cylinder by loosening the two brake pipe connections while slowly depressing the brake pedal to full travel, then tightening up again.
Ok, so we got all the plastic off from around the Master Cylinder and hey presto... the master cylinder has a bleed nipple!!
No need to undo either of the brake pipe connections.
The guy I was doing it with was a very experienced mechanic and he said he'd never seen a bleed nipple on a master cylinder.
Anyway pressed the peddle opened the bleed nipple and sure enough lots of air rushed out
repeated a few times and brake peddle and braking back to normal.

 

Ive never seen that before and a quick look on ebay and there it is.