Today I bled an Avid Elixir 1 after it had gone soft over the winter. I followed the directions in the Park Tools Big Blue Book completely, used an Avid bleed kit and opened a new bottle of DOT 5.1. The bleed went well and the lever had good pressure afterwards.
I went out for a couple of hours and came home and thought the brakes seemed softer. I stood the bike on its rear tyre and pumped the brake 10 times and by the end the lever was against the grip.
I checked both plugs and they seemed to have a good seal.
Any ideas on what I could have done wrong or could be wrong with the brakes to allow significant air into the system?