Hello Mike,
Graphic 66 is right, these are the classic symptoms of a bad brake master cylinder. Since this is a dual system, the pedal reaches the secondary circuit near the floor.
During hard braking the fluid pressure will keep the seals flared against the cylinder walls in the master, however under light pedal pressure the seals relax away from the brake cylinder walls loosing their seal and the pedal slowly creeps downward until reaching secondary circuit near the floor. If you look carefully under your master cylinder where it meets the booster you may see signs of some brake fluid leakage. Brake fluid will also begin leaking into the booster often times.
Rebuild kits are available, but most people believe it is not worth the time or risk to save about $50 or $60 dollars rebuilding one. New master cylinders can be had for around $100 ovften times.
Be sure to bench bleed your new master before installtion. Conventional brake fluid is excellent paint remover so don't spill any on your car's finish!
Mercedes recommends that brake fluid should be replaced every two years! Brake fluid absorbs moisture from the air which internally rusts your brake cylinders and calipers! This moisture can also cause complete brake failure if it boils during very hard braking. In such a case, the boiling water will fill the system air and the brakes go away!
Joe Alexander
Blacklick, Ohio
