first you need to measure the clearance between the pins and small end bushes in the rods. make sure all the pins are the same size and check all the rods for clearance. if thats all ok, the piston issue is probably just small burrs where the pin goes through. u can either use a file and sandpaper, or a small hone like you would use for a brake master cylinder. the pins should push through the pistons with ease, if you have to tap/press them there is something wrong.