Also, maguras are designed to take huge pressures - like when you squeeze the lever for example. I don't know exactly how much pressure there will be in the system under hard braking but I imagine it's a lot. I.e. Water is probably not going to damage the system. However, having a ride cut short (and having to walk home) because your brake is locked on is not ideal Syringe bleeds aren't hard to do at all, but they're not easy to do well. The bath bleed is genius really - it's so easy to get a perfect bleed. For me that's the best argument for using water in a magura. Unless you're rich and like making a mess, then you can bleed maguras with a vat full of proper mineral oil. Anyway, yeah I've bled a brake with WD40 - just spray it into a bowl and then use the normal syringe method. But it's not very good...