You want to re-pot it on the flop. Very often, a decent player will fold two pair to a check-raise (I would). For this move to work, the opponent has to fold 1/2 the time. I think that will normally be the case on that flop. If you lead out, it's very easy for an opponent to think you are stealing the pot with a marginal holding (especially if you raised preflop). A check-raise has to be respected, though.

If you check-raise and someone moves in, you better fold. You are probably way behind. At least at this point you know where you are and will not loose any more money. The more difficult situation, actually, is what to do on the turn if your raise is flat-called. Most of the time I'd give up on the hand, but, with a good read, leading out again on the turn might be correct.