master cylinder fluid level

[paulbrgr9999]

I was just routinely checking things and opened up the brake master cylinder resivor and noticed a large discrepancy between the two levels. It appears that the rear brake resivor is lower than the front. I checked under the truck front and back and behind the wheels and the lines and cant find a leak.

I included a pic. I cant find a fill level mark on the master cylinder or in it so how do I know how much I should add? Should I fill it to the top?

paul