Had this problem one. One of my calipers was frozen, causing a drag on the brakes and heating the fluid to boiling. Once the brake fluid boils, you have no brakes.

Possible causes:

1. Frozen caliper - drag causes brake fluid to heat and boil
2. Old brake hoses with blocked passages - will prevent oil from returning, which in turn will casue a drag on the brakes and eventually cause the fluild to boil

Check each caliper for frozen pistons and replace all rubber brake hoses.

Also, brake fluid should be replaced every two years. Brake fluid is hygroscopic and with time its boiling point drops. Also, the water in the fluid promotes corrosion/rust which will tend to freeze the piston in place. New brake fluid's boiling point is higher and you have a slightly higher margin of safety in case of a dragging brake.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold