If you pedal sinks all the way down to the floor you must have a good vacuum which means its not a vacuum line/hose or brake booster issue. About 3 years ago I had a friend that owned a 89 gtr his brake master cylinder leaked after sitting all winter in a non insulated garage. The best way to confirm you have a leak is to check fluid level in the reservoir. If its low shine a light on the brake master cylinder and see of its wet if it not dripping brake fluid top up the reservoir, and spray down your master cylinder with brake cleaner. With the car off slowly pump the brake pedal with out bottoming out on the floor. Do 5-10 pumps and then hold pedal down, If the pedal doesnt drop at all when you hold it down with car off, i suggest starting the car and repeat to be on the safe side. If it drops at all in both car off or car on bleed brake system to be on the safe side. After systems has been bleed, recheck brake master cylinder for any leaks. If its dry and your brakes are firm, holds pressure.best case is the seals shrunk when it was cold out allowing the fluid to leak out while in storage over winter. The best way to confirm this is to test the brakes on a warmer day so that you don't end up losing brakes on the public roads if everything checks out.