Hi Dougie
As above and…
Which callipers? if they are 1990s models then only part of the following is true.
If they are the ones fitted to Tonti era bikes the here you go;
Have used this in the past when chrome plated pistons have seized.
1: Split calliper.
2: Lay the halves piston side up.
3: Gently warm the calliper (rag soaked in boiling water, hair dryer etc). Too much heat (blowlamp) has potential to damage the calliper body.
4: Apply brake fluid to the piston-calliper.joint and leave to soak.
5: Repeat a few times and try to start slight movement in the piston.
6: Clean, fit Teflon coated pistons and new seals etc.
If they are the later callipers you may struggle to find seal kits. Brembo stopped supplying them because of possible litigation should a rebuild go wrong 
This is not to say that 3+ above won’t work.
There is a rebuild service available for the later models. The link is somewhere else on the site.
HTH Good luck
Steve