1973 Dart brake failiure

Hey all, It's me yet again.

After I drove my dart back home from its storage location and sat overnight, the brake pedal felt alittle too easy to press, and the brake pressure got worse and worse until now it won't even hold the car in idle while in drive. The brake warning light comes on Intermittently aswell.

I checked for fluid but I dont see anything around any of the brake components, and my next logical step is to check the MC for brake fluid, but I've never touched this system before.

I looked at the service manual and either I just don't understand it or it doesn't describe it very well but after reading the brake section I am still confused as to how to go about doing this.