Brake hoses and lines connect the master cylinder to the calipers. Over time, brake lines can eventually wear out, develop cracks, cause leaks, and even explode due to fatigue. They should be checked whenever servicing your brakes.
When you hit the brakes the master cylinder converts the motion from the brake pedal being depressed into hydraulic pressure and sends that hydraulic pressure out for distribution. If your brakes are starting to feel spongy it could be the master cylinder.
If your brake pedal is getting harder to push you may have a vacuum leak in your brake booster system. Used on power brake cars, they accept a vacuum hose from the engine manifold, and use that vacuum pressure to assist in the movement of the master cylinder.
633CSi (1982-84), 633CSi (1982-84) Note: For vehicles produced from 09/1982 635CSi (1985-88) Note: For vehicles produced from 09/1982, 635CSi (1985-88) M6 (1987-88), M6 (1987-88) Note: For vehicles produced from 09/1982