So, I've lost all pressure in my brakes, and from what I can tell, it's leaking out of the cap of the master cylinder. The lip on the cap is slightly bent out on one side, could that be what is causing it?

Thanks.

Whats the condition of the gasket in the cap? If the gasket is in good condition and the lid is still flat behind it helping to make a good seal it should be okay. Assuming the cap isn't damaged so much that it is not holding the gasket down flat against the top of the master cylinder, I'd say you need to get a new gasket.