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Why do I keep destroying O-rings?

I'm very, very distraught.

I am trying to rebuild the right rear brake caliper on the 90 cq. The
piston is a Girling screw-in. Every time I screw the piston in, I take a
half-inch notch out of the o-ring that rests in the cylinder groove. I'm
not completely severing it. This is before any hydraulic pressure is
applied. And on the three rings I have used, the notch is identical!

I have absolutely no idea why!

Thing is, the piston is as smooth as glass. Really smooth. And I'm using
buckets of brake cylinder paste.

My only two hypotheses:
1. There is a microscopic imperfection on the cylinder.
2. The friction between the cylinder and the ring is sufficient to take
a chunk out.

These hypotheses seem incorrect. Am I missing anything?

A bad batch of rings? Any way to buy these rings at a lower cost?

These kits are getting expensive.

I should have stayed in bed today,
Ted Harlan