Is one dumbbell and a pull-up bar the [/i]only[/i] weight training equipment you have access to?
... for what? Bigger pecs? Stronger pecs?
If you're after bigger pecs, you'll need to gain muscular bodyweight and you'll eventually need to lift heavier weights. If you're after stronger pecs, you'll have to lift heavier weights immediately, or at least have access to more equipment.
I'm going to assume that you can knock out a good number of standard push-ups already, so those will be an inferior exercise choice. If you can figure a way to do dips (on chairs or something similar), that would be the best bet so far.
That's because, outside of some push-up variations and dips, there aren't that many strictly-bodyweight exercises that can target the pecs. If you have rings or straps, that opens some more variety, or a basic free weight set would obviously be the most useful.
Do you have to stay in a weight class for your Muay Thai?