Grinding is bad. You may be damaging the rotor which may mean you will need to have it resurfaced or replaced.
The calipers MUST be lubricated. Or more correctly they sliding surfaces the caliper rides on are lubricated with high temeprature caliper grease. You should do this periodically, like every other oil change or so.
These calipers are maybe the easiest I have ever worked on to remove and clean and grease so no reason not to greaset them. Takes maybe 30 minutes per side including jacking the car up and cleaning stuff off. 2 bolts and lift off and clean, easy as that.
If the pistons themselves are sticking then you have a different problem which will probably require a minor rebuild of the cylinder, essentialy clean out the rust.
Check your rotor bot the outside and inside one. There may be metal to metal wear which is bad. Fix it soone before it costs you a rotor.
Good Luck,
Jim Henderson
