Kefer,

A simple way to test for leaks in the hydraulic system is to press the brake pedal down hard (two feet will help) for about 5 minutes or so. Your pedal could go to the floor or sink a little, and if it does, you will see hydraulic fluid leaking out somewhere. That is where your problem will be.

On other hand, if you don't see leaks, and the pedal sinks, you have air in the lines. You will need to bleed the brakes.

If you determine that your reservoir is the problem, you probably can order new seals. By the way, a rebuilt master cylinder may not come with a reservoir, and you will have to reuse your old one. It might pay to replace the reservoir seals if your master cylinder is fine.