Yes you will, however the IFL is in the international terminal and it will likely be a long walk to the gate for your SFO-ORD flight. You won't have to reclear security though..so if you've got plenty of time you can check it out. The IFL at SFO is one of the best lounges UA has anywhere. You will need to show your F class ORD-BRU boarding pass to get in.

In the future.. use the consolidated United forum. I don't even know why this pre merger forum is still open.