[KiWinUS]

Tim brake seals should NOT swell up. If they did it could cause the pistons in either master &/or wheel cylinders to stick. I have seen this with AP racing master cylinders and worked with their engineers in England to resolve the issue. They had made a batch of seals with wrong material.
I have used several of that same master cylinder with zero issues. I dismantle all new master & wheel cylinders clean them polished their bores with a tool I have made to hold a strip of scotchbright that I use with high speed drill motor clean again. Paint with brake fluid resistant paint. Clean once again & assemble using nothing but brake fluid as a lubricant. All of our race components are also assembled with brake fluid only as a lubricant.