Master cylinder - low fluid in rear reservoir?

[DarrenX33]

Having a strange problem with my master cylinder. Upon pulling the the car out of storage I found that the brake pedal was going to the floor. I pulled the cap off and found that the rear reservoir was near empty. The funny thing was that when I pulled the cap off a lot of fluid spilled over and out of the front reservoir, almost like the rear fluid was going into the front.

I do see a small drip line down the booster. But when I separate the master from the booster it seems dry so I don't think its coming out the back of the MC when I push the pedal. Wheels cylinders look dry and nothing visible on any of the brake lines running to the rear?

Proportioning valve???