HK sells this and the disk assemblies separately. You have to get this and the disk to build a full system. You will also need a tank to store pressurized air. The system is pneumatic - works well. Didnt try it on hydraulic.
It is a proportional brake valve and it is pneumatic. It works with the brake assemblies sold elsewhere in the site, but there is no system, you have to make fiure it out and make your own. Interesting products, allowing you to add brakes to your plane.
In case people have missed the few correct remarks above - it is indeed hydraulic, and only hydraulic. It's a piston that applies force onto a body of oil. The stronger your servo, the more brake pressure, exactly like in a car (although there the servo does most of the work when boosted). Of course the piston will compress any remaining air in the system, hence the need for a certain travel, although ideally you should try to remove all the air (as it is far more compressible than oil).