I recently had this on my mk4 golf, people were telling me it was the master cyclinder.

It wasn't though. Below my fluid reservoir there are 2 bleed nipples,
I bleed the master cylinder via the 2 nipples and the pedal came back.

You may have let the level fall to low and now have an air lock in the master cylinder.
Check to see if you have bleed nipples on the master cylinder and try bleeding it from there



Also not sure if its possible on your car, but have you got the calipers up the right way? i.e bleed nipples at the top,
Ive heard of people fitting the wrong caliper to the wrong side of the car with the bleed nipples at the bottom, this would also make it impossible to expel trapped air from the system