Calipers

This really depends to the condition of the caliper.  One may try to uncover the piston dust cover, not that it would be partially uncovered any way, since you are experiencing  sticky pistons.

The pistons can be carefully cleaned with the finest wet and dry emery you can get, if these are rusty at worse, or coated with dirt and grime.  To wet, the emery, use brake cleaner.  Do this for all the pistons, rinse off with brake cleaner and to help with lubrication, either smear the pistons with brake grease or apply a liberal coating of brake fluid. 

Work the pistons in and out several times, taking usual care the pistons are not over pumped, that, they pop-out of the caliper; if it performs as it should do.  Fit new dust covers and re-fit.  I am assuming the caliper body is cleaned off and greased with high melting point grease such as copperslip or a more newer version made of zinc (I do not know of its trade name), in between the brake pad edge and caliper rest.

If the condition of the pistons are grooved; by the time new ones are fitted, the caliper will cost almost as much, if not more, than a re-manufactured one.  In this case, I suggest getting a re-manufactured.

The URL address included, at last enquiry seemed competitive and they knew what they were talking of.

http://www.brakesint.co.uk/    You will need to mention, if the caliper off your vehicle is serviced with one, or two brake pipes to the caliper.  The caliper from them, I think, is an exchange unit, for which there is a refundable surcharge applied.

Cheers,

Brakes International were quite helpful and very efficient.  They supplied 460 Calipers to me recently (double pipe), and I never sent my old ones back to get the £40 refund (leff packaging).  I don't think it was my calipers (had to replace the master cylinder), but they do need new seals and rubbers.