Definitely O-ring in first
Last year I purchased an RV7A from the original builder. There were numerous issues I've been force to address in my first year of ownership, one of which was loss of brake because someone installed the brake pucks backwards. Fortunately, this was discovered during an engine run-up when I was alone at the hammerhead while holding the brakes rather firmly. (The plane does not have a parking brake.)
However, even though no one was around me, the feeling of one peddle going fully to the floor and the quick twisting moment of the aircraft around the good brake wheel was enough to give me great concern! Pay close attention to how you re-insert the brake pucks. Do it wrong and you or someone else will regret it!!!