After brake light on instrument panel stayed on, I checked fluid level in master cylinder on my ‘72 cutlass (disc/drum brakes) and front was full but rear of MC was almost empty. I filled the rear almost to top. Now I am not sure proper way to reinstall MC cover. I assume you depress the bellows of the diaphragm on the cover first and make sure cover is secured by the metal latch. Can I pull the car into the garage to get it out of the hot sun before reinstalling the cover?Also, the cover has been removed for a couple of hours now while I have been searching for solutions to my low brake fluid problem. Will I have to have the brakes bled now that I have added fluid?

 

You should never leave it open especially if it's really humid outside. Brake fluid absorbs water. Yes, you depress the bellows and clip it on. I would put it on before moving the car, you don't want debris from the hood insulation getting into the MC.

There must be a reason the rear was empty. It could be a bad master and brake booster where the fluid was sucked into the engine. Or it could be a leak at either of the rear brake cylinders, the rubber brake host above the rear axle, or a small leak in one of the metal lines going to the rear or to the rear brake cylinders.

Based on your questions, I would hire a professional to take care of your brakes. The consequences of brake failure are serious enough that you shouldn't be working on them if you don't have any expertise.