Hi Sebastian... first thanks for the info, that is really helpful!
I ordered two of the 1000ml kits from Freestyle in the U.S. back in April 2011. When I went to open the second kit, the Part C dev was already a deep dark purple (not really red, but most definitely not yellow). That would suggest to me that the bottle was not airtight and that the Part C should be replaced with fresh chemistry. Since it has only been 4 months since I received the kit, it seems like there might be a problem with the way the kits are assembled if the developer Part C is not lasting for the 2 - 3 years that Maco indicate it should.