Quick guide - this never fails when done correctly.
Attach a bleed pipe to the slave cylinder, and put the pipe into the presta fitment of a track pump, or an air line. Remove bleed screw from the lever.
This bit can get messy, go slow at first, and then use a rag to catch all the flying fluid/water: blast air through the system to clean it all out nicely.
When this is done, put a couple of drops of chain lube on the seals in your lever, pull it a few times to work the lube around, and then wipe off the excess.
Turn all TPA off.
Re-bleed with water/water-antifreeze mix.
When its all bled correctly, put the TPA fully on and depress the lever (elastic band (Y)" ).
Push one of the slave pistons in by hand, and put a few drops of lube on the other one (WD40 also works ok). Then repeat for the other cylinder. Take the TPA off, remove elastic band, and pull the lever a few times to work the lube in. Wipe off the excess.
If all done correctly, this will have your brake feeling better than new (Y)