VW Quantum Syncro master cylinder. The master cylinder is responsible for converting your foot pressure into stopping power for your vehicle. If your brakes feel spongy, or the pedal is progressively getting softer it is time to replace your master cylinder, before your Quantum no longer stops at all.
Flush fresh brake fluid through your brake lines when you change the VW Quantum Syncro master cylinder. Never try to reuse old fluid, over time it draws moisture from the air and then can't handle the heat generated while stopping. While bleeding your brakes after installing this master cylinder, make sure to inspect your brake pads to make sure your new master cylinder has brakes to work on.