In times past I have freed off siezed calipers... Usually the outer seal is just for keeping dust out, I have used WD40 with the small diameter tube attached to the nozle, prised the top edge of the dust seal from it`s groove and sprayed a good amount of WD into that location, I guess you could leave it for as long as you want and repeat the process as many times as you want, but I was ever in a hurry.. (-: I would then force the piston back into it`s bore using whatever suitable lever was available... Though best if the other pad is in place if you are levering off the disc which I have done many times... These days you can by all sorts of gizmo`s to achieve the same ends, I have an old joiners clamp that is put to use on occasion.
With the piston forced back and the other pad still in place when the brake pedal is pressed hard the piston will be forced out though best to have whatever you used to lever it back, still in place as if you pump the pedal too many times you could end up jaming the piston up against the disk with no means of forcing it back...!!! At this stage another spray of WD40 can help before forcing the piston back and forth a few times to free things off.. I have also removed the piston completely to clean up the corroded ring above the seal in the caliper, and if you arrive at that stage I think you can buy new stainless pistons, to cut down on refurb costs. If you get to that stage, it seems best to buy a new set of seals too..
The seizure can be caused by either the piston becoming corroded or the small area/ring in the bore of the caliper that is on the outer side of the fluid seal expanding slightly and gripping the piston... of a combination of corrosion of both those components. I have cleaned both to some degree and returned the brakes to operation on my cars over the years.
As long as the area of the piston that makes contact with the fluid seal is still clean and free of pitting, my brakes operated well, having used similar processes over quite a number of years in my impoverished youth.. (-:
Of course today you will be well advised to buy whole new calipers, though there are folk who will refurb your originals with new seals, pistons and bleed screws.. and will paint them any colour you wish too..(-:
I think there are a number of folk supplying upgradeed multi piston calipers and discs vented or otherwise for Morgans.... My mid 80`s +8 is still running original discs and calipers, though I have replaced the caliper seals once in the last 20 years and circa 24k miles during my ownership
I hope you have success with whichever route you take..
