baattery helmet locked on battery post

[mk31755]

Hey guys I could use some help.  I didn't know what section to put this in.  I have an old 56 healey and the batteries are behind the two front seats.  There are two six volt batteries.  They are connected to the battery terminals with helmets.  The helmets are brass and the terminals are lead.  I can not separate the helmets from the terminals.  I've tried everything. Any suggestions?? 

                MK