Dreaded lid leak on new master cylinder...ideas?

You can always completely flush your brake system and replace the fluid with brand new DOT 5 fluid. DOT 5 is silicone based and non-hygroscopic (will not absorb moisture from the air). DOT 5 is also non-caustic and much gentler to your paint finish should you spill any - but you should still clean it from any painted surface quickly.
Read about the different types of brake fluid: Classic Car Brake Fluid