Jim tells you that Brian wants to call a truce. Johnny isn't interested and wants to see Brian.
Brian is waiting at the docks, so follow your GPS to his location. Stay close to the guys to hear what they say on the drive there.
It's A Set Up
Quite obviously (or was that only obvious to me?), Brian's meet was a set up, and he's turned a load of Lost members against you in his own faction. Van and bike loads of them have been sent to kill you, but luckily you're behind some pretty damn good cover. You can just stay where you are for the majority of the firefight and take out the bikers and then foot soldiers who come out of the vans as and when they arrive. If you have an RPG or an AK-47 you'll be able to take them out at long range, otherwise just wait for them to get close.
Eventually enemies will start climbing over the wall from the left, so make sure you take them out. Brian will then appear and complain about you starting a civil war, then take off on his bike.
Don't Let Him Get Away
After the cutscene, jump on your bike and chase after Brian and his crew. Get close to them and blast them away with your shotgun. Once you've killed all of them, you'll be told to meet up with Jim, although usually he'll drive towards you anyway.
A cutscene will start back outside the club house where Johnny says the Lost simply need to be brothers and not numbers. Then he says he wished they'd pinned Brian down and he'll be back soon. I thought I'd just killed him, but obviously not.
- Stay in the cover you're provided with at the start of the mission. You barely need to move, apart from when the chase scene kicks in.