It seems as though the plunger in the master cylinder isn't retracting after use
Occasionally when motorcycles are stored you can run into this problem. Older brake fluid along with a bit of moisture and oxygen can contribute to a buildup of material inside the master cylinder bore where the plunger goes. After pulling the brake once the plunger sticks and doesn't retract.
You can remove the plunger from the master cylinder by removing a circlip that retains the plunger
Remove the plunger and clean the master cylinder. Use water and no petroleum based cleaners. The seals are designed for DOT 4 which will mix with water, petroleum products create a high risk of damage to some of the rubber seals and o-rings. You can use a que tip to get in there a bit more or just a paper towel wrapped around a chopstick. Once you have cleaned everything. Reassemble with a bit of DOT 4 brake fluid as a lubricant for the plunger.
You will obviously need to bleed your breaks upon completion. These little vacuum pumps are cheap and make the job considerably easier rather than sitting and pulling that lever hoping your going to get that initial air out.
Here is a parts explosion of the master cylinder. You didn't indicate 750 or 800 or a particular version so I just used the 750 for this example. They are all the same master cylinder essentially.