A bit more info?
Can you give us a bit more info? Are there any particular situations in which they work or don't work.
A hard brake pedal can be a result of:
1. Buggered vacuum pump. Check line from vacuum pump to air filter. Does it have oil in it? When you pump the brakes as you drive, does the vacuum boosting effect reduce to nothing after a couple of pumps? Solution: Spend $50 on a rebuild kit for the vacuum pump and fix it yourself.
2. Buggered rubber brake hoses. If they are swollen, then the fluid may not be getting through properly. Check the flexible lines (run between caliper and car body) for age wear (cracking etc). Solution: spend about $100 on new brake hoses - just replace the lot!
3. Metal brake pads. Solution: Stop being tight, buy OEM or genuine Mercedes brake pads if you want geniune Mercedes stopping power [

]!
By the brake light, do you mean the pad indicator wear light (circle with dashed lines)? Or do you mean the one that says "BRAKE"?
1. If it's the pad light, then you have a short. Solution: Spend the $5 on new wear indicators and carefully install them, being sure not to damage on installation (or you'll have the same problem again).
2. The brake light. Check the hand brake light switch. Check the brake fluid level. Check whether the float sensors for the brake fluid level are working.
Hope that's some help.