If the brake fluid is still in the reservoir, then your master cylinder is shot.

If the fluid is gone, then you have a leak in a line or in a caliper. Easy to find - just look for the wet spot.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport