Postgres Streaming Replication: How to re-sync data in master (with standby) after failover

I am absolutely new to postgres 9.1 replication mechanisms. I have managed to set up streaming replication between a master and standby postgres (Both have x number of records). At one point when the master server fails, using the trigger file mechanism the standby takes over, and accumulates additional data (say now has x+y number of records). Now when the Original Master Server comes up, it still has x number of records (which is now the new master). Is there a way to fetch only the delta, i.e. newly added 'y' number of records from the standby and restart as Master. Or do I have to always take entire base backup?