i have just had the rear calipers replaced on my 155, the bleed nipples on both had been snapped off (one before i had the car , and i snapped the other)
one important thing to look at is the condition of the slides that the caliper 'floats' on. if the rubbers have perished then allsorts of crap can get into the slide and it will seize.
The only way i could get the 'pin' out was to remove the caliper, then unbolt the carrier and place it in a vice , then i heated the area around the 'pin' with a blow lamp. After about 5 mins the 'pin' came lose and i could remove it.I then cleaned out the hole and pin with sandpaper , relubed it, put on a new rubber.
Once on the car the brakes were ssooooooooo much better and the feel of the pedal was firmer.
As a result I stripped all four calipers in the same way, all four had at least one 'pin' seized or very stiff .
I have also fitted steel braded hoses while the brakes were stripped, £52 for a 6 hose set.