I was driving back from work and when I hit the brakes to stop at a stop sign, they were very soft and the pedal sank to the floor - felt like the master cylinder went out.

I pulled into a gas station, stopped the car and waited for a few minutes. Then I started the car and pumped the brakes and after a few times, the brakes were back to normal.

What could be happening? The brake fluid is need of a flush - it is black in color. I recently checked the brake pads and they all are good. I checked to see if there is brake fluid leaking anywere and there was none.

If I just bleed the system, would I be ok or do I need to go in for a new master cylinder?

Thank you for your responses.

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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

If they didn't come back until things cooled off, then, yes, it needs a flush and fluid change. Brake fluid collects moisture over time which makes the fluid easy to boil when you need them most.

Also, if you have the dust shields, or "Kleen Wheels" inserts in your wheels you should remove them for safety's sake to prevent overheating in the future.

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02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

I would bet on failing master cylinder seals. You need to check for fluid leaking between the master cylinder and the booster. If you see any, it's a sure thing that you need a new master cylinder, and possibly a booster.

Sticky caliper(s) and/or bad rubber brake hoses can cause brake fluid to boil, and you will lose your brakes until the system cools down. I had this happen on a 450SL, and it was scary trying to slow down from highway speeds for a stoplight. I didn't smell anything funny before, and the car didn't pull to one side or the other before it lost the pedal. The calipers didn't leak either. Degraded brake fluid can boil at much lower temps than fresh fluid also. I rebuilt the calipers the next day and replaced the brake hoses as well. The caliper kits cost around twenty dollars each and the hoses cost around the same. Just my .02 .

I will flush the system as soon as possible. And later when I get time, I'll probably change the brake hoses.

__________________
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