A few years ago we had a customer that had been struggling to bleed their brakes on a 1.3 Series 2 Fulvia Coupe. We had spent hours, literally, on the phone talking them though things to try in order to find the cause - old hoses, crushed pipes, blocked master cylinder ports, servo pushrod lengths. All the usual things.
We didn't find the reason (over the phone), so proposed on of our technicians should pop down and try to sort it. There were a couple of other jobs he wanted doing, but trying to sort out the brake bleeding was the main one.
As soon as the rear wheels were removed the problem was obvious. The calipers had been fitted on the wrong sides, so the left caliper was on the right and vice versa. That causes the bleed nipple to be at the bottom on the caliper, which prevents the air from escaping (air being lighter than brake fluid). A few minutes work later, and it bled nicely immediately.
I've mentioned that story in passing a few times, but never thought it would be useful, but this week I had a similar experience of trying to help a customer (a mechanic of many decades experience) with a brake bleeding problem. As the ideas were running out, I mentioned the bleed nipples have to be at the top... "Hold on, let me go and check." was the response.
Needless to say, they were fitted upside-down, and sorting that the next day immediately allowed a successful bleed!
Calipers, like household radiators or the cooling system on a car, need the air to be let out at the highest point available.
TLDR; if you are struggling to bleed your brakes and achieve a good pedal, just check that all the calipers or wheel cylinders have the bleed nipples at the top!