You can force a "master" keeper failover using the stolonctl failkeeper
This commands forces a keeper as "temporarily" failed. It's just a one shot operation, the sentinel will compute a new clusterdata considering the keeper as failed and then restore its state to the real one.
For example, if the force failed keeper is a master, the sentinel will try to elect a new master. If no new master can be elected, the force failed keeper, if really healthy, will be re-elected as master
To avoid losing any transaction when using asynchronous replication take a look at this recipe: