Taking hot backup of slave node in postgres (master-slave config with repmgr)
At least on 9.3, you can run pg_basebackup against a hot standby replica, and that's the approach I would recommend. To do this, you must have postgresql.conf set appropriately:

max_wal_senders = 2
hot_standby = on


You didn't specify your version. If you're on a version that can't do a pg_basebackup from a standby you're pretty much out of luck - you need to do a logical dump, dump from the master, or stop the replica for the backup.