This seems to be hit and miss, I've bled a lever in water and had it do this after a few months. I've also bled a lever and it's been fine for a year or two.
Others have had the same lever for longer than that, running water with no issues.
http://www.tartybikes.co.uk/rim_brake_spares/magura_hose_clamping_blocks/c5p10026.html
This, a vice and a hammer.
Or you can make your own out of some wood using a drill and a saw.
Adding to the 'things you convinced some kids about trials bikes', I convinced some kid that the TPA dial was used to set how far you pedal kick, and when you let go of the lever in a back hop that's what shot you forwards.
Now's the time for them to pave the way for an external BB system in trials, whilst creating a screw thread compatible design, and giving the cranks more offset.
Shame that'll never happen.
I just need to sort the chainline out and it'll go.
A new set of pedals and a new brake lever would finish it off. I'll spray the forks black in summer.
If anyone has a set of inspired high rise bars in black knocking around, I'll have them.
Want a tutorial for bath bleeding? Look in my signature.
As above - undo the hose fitting a little bit. That's not in my guide because I made it before those brakes came out.