Using 1/2 of mobile duplexer for RX input only?

I would like to use 1/2 of mobile VHF duplexer as a 'filter' on the RX radio as part of a repeater setup. The radio is RX only and will not be set to transmit at all. I noticed most BP filters only do low db isolation where duplexers do like 50-60db.

Can the unused freq side be just left open/not connected?