In the event you think there is no brake fluid left in the rear reservoir, here is how you check it.
Using a 5mm hex key, undo the cap screw just to the right of the window.
WARNING: The nut the screw goes into is a square captive screw, but captive only in two dimensions! If any force "inward" is applied, you will push the nut into the battery box!
First, loosen and undo the cap screw while at the same time, pulling gently on the bottom of the cover so that the unscrewing action isn't pushing the nut out of it's "almost captive" mount.
When the cover comes off, you will see the small plastic tank/reservoir and if it's empty it'll be obvious, as it will if it's completely full.
If you need to fill the reservoir, get plenty of rags to support it and catch any spills. Use a No2 Phillips screwdriver to remove the cap and diaphragm, top up with the specified fluid, replace and secure the cap and re-fit the reservoir, the support stud and brake hose tube support in place and secure with the 5mm cap screw.
Make sure you have all parts loosely in place and then position the screw and gently engage it with the captive nut and take up the slack. Check all items are in correct alignment and secure by tightening to medium "hand tight". Ensure the tabs at the rear of the cover are positioned under the upper cover as you secure everything away.
And that's all there is to it.
Hope it helps.