I decided to clean the brake check valve & hose but thanks for clarifying.I aM glad it might help you. However, I want to apologize that after reading my suggestions I might not have been as clear as I could have been, as others have been with other's answers. And I want to be more on target:
The problem IS THE booster VACUUM CHECK VALVE loosing it's sealing capability in part, intermittently, or entirely by getting oil contamination caused by a "going bad" or bad PCV valve.
I would recommend replacing (or thoroughly cleaning both parts. Engine decreasing cleaning additive solvent, break cleaner (short length of time soaking) water clean out/drying in my case)) REPLACEMENT of both parts better/safer). In my case I cleaned & so far so good.
Again booster check valve and PCV valve NOT just one is the problem in this case of intermittently loosing break/pedal going/spongy/sinking break pedal!!!!
I feel certain that this is not a hydraulic brake issue. There are no brake fluid leaks on this car. A hydraulic leak will not recover an hour later on its own. A brake booster will not repair on its own either, That leads us to vacuum supply, or lack of, therefore cleaning the vacuum valves makes sense as a repair attempt. So far so good.