overpair on 4 flush board--bet the river?

[captain_swing]

I would really appreciate some discussion on the rationale for betting vs. check calling the river and what factors would swing the decision. My default play is to check call but I'm worried that is a leak. Opponent will have a flush about 35% of the time (if he is holding random cards). My thinking is he will have a harder time calling with hands I beat than with bluffing. If I bet do I fold to a raise? This opponent was loose passive though, so maybe my thinking is wrong.

Party Poker 3/6 Hold'em (9 handed) converter

Preflop: Hero is MP3 with A[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/diamond.gif[/img].
<font color="#666666">4 folds</font>, <font color="#CC3333">Hero raises</font>, <font color="#666666">1 fold</font>, Button calls, <font color="#666666">2 folds</font>.

Flop: (5.33 SB) 8[img]/images/graemlins/heart.gif[/img], 4[img]/images/graemlins/spade.gif[/img], 3[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets</font>, Button calls.

Turn: (3.66 BB) Q[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets</font>, Button calls.

River: (5.66 BB) 8[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, Button checks.

Final Pot: 5.66 BB