@SpookDog the cup should be facing the brake pad I've never seen flat side facing the pad although I don't doubt it exists the cup keeps little contact area so it's less likely for corrosion to glue the pad to the piston EDIT- emphasis on the "should"
@Bananper Check all your lines and look for evidence of a leak, if it isn't from your master cylinder or hydraulic lines then dismount your brake caliper and check it over.
@finnerz89 @Calum it's not possible to get the other part (not sure what it's called) off the slider, tried knocking it off with a hammer but it really won't come loose... Never experienced something like this.
I don't really want to risk breaking the part of while knocking it off before i know of a caliper that's available