Frustrating brake issue

Good morning! I am rebuilding the front disc caliper on my 74 and have run into a problem. After my first attempt, I bled the system and everything seemed to go fine. Then after pumping the brakes a few times, fluid dumped out from the caliper. I took everything apart and discovered the inner piston seal had a chunk taken out of it. I inspected everything, the bore looks good, the seal groove is clean. The piston is brand new. I chalked it up to user error and tried again. Same problem!! The funny thing is the chunk taken out of the new seal was exactly the same as the first one! I'm stumped! The piston does seem to go in hard even though I am soaking everything in brake fluid first, so I suspect when I tap it past the seal it is catching and tearing a chunk out. Any tips or ideas?

You need to put the outer piston in first, then the seal, then push the piston backwards from the centre of the caliper towards the outside over the seal. Piston has a rounded edge on the closed side and a sharp one on the open side.