Thanks for the answer! But I don't think my problem is similar, because my breaks are great after I bleed them, and as long as I ride regularly they keep like that for weeks. The problem I notice is when I don't ride for a few weeks, they degrade quickly and become very soft (I can't even break with 100% before hitting the handle, so I have to pump them previously, or tie them overnight to get some air out).
There is no leakage of oil anywhere, nor is anything damp...
Also, the brake hoses expanding with age or whatever is not consistent, because everything works great after bleeding, and then becomes soft as the bike sits for a few weeks. So it's definitely something happening over time, like air getting in somehow.
Thanks for the answer, this one seems right on point - since my brakes are doing well after bleeding them, and degrade after a while, seems to point air coming into the system somehow, not so much that the hoses are stretching due to age, etc.
About brake pads: that would make sense if the softness is permanent, but it's not - if I bleed them, they are good for a while. I will check the pads anyway though.
My concern about the caliper seals is this: how can air get inside a pressurized system, without oil getting out ever?
So it has to be master cylinder? Or does the same apply there? I can't really make sense of it all because I don't have any experience with brake systems :|
By the way, I just checked and OEM Kawasaki hoses are ~420€ JUST FOR THE FRONT END! Oh man, I wasn't expecting it to be that expensive :/ are there good quality solutions from other manufacturers which would be cheaper and widely used perhaps? :D