We want that out of there. It may be better for those cars to leave the battery installed and make sure you have it on a trickle charger. You really should change the brake fluid. It is hydroscopic. This basically means it loves to absorb moisture.
Moisture sitting in the equipment over the winter means corrosion.
This means brake problems. Porsche, along with all car manufacturers, recommend changing the brake fluid at certain intervals. However, as was the case with the engine oil, storage allows contaminants to remain in the system without being burned off or flushed. Thus, it is important to put the car away with fresh fluids. If the car has been used on the track this is even more important.