Brake caliper repair help

Last night decided to overhaul the front brakes on the 300GD 1986, so armed with new discs, pads and a caliper repair kit, i stripped down the passenger side to find that 2 of the pistons in the caliper had seized solid, not a problem just a 2 minute job i thought, just clamp moving pistons and pump the brake pedal, 2 minutes turned into 2 hours before i finally popped them out!.So now they were all out and cleaned up, the easy part is putting it back together!!hmmmm. New seals on the pistons no problem, slide pistons into caliper no problem, fit new dust covers over pistons.....big problem, they refuse to go on! I have tried everyhting now from clamps to grease, to try to sqeeze them on, but with no luck!
Does anyone out there have a tip for fitting these ?
They are genuine Mercedes parts and they look identical to the ones i have taken off.so they must be the correct parts.