1982 Mercedes Benz 300sd bleading break fluid

Tiny
JOSIAHT
  • MEMBER
  • 1982 MERCEDES BENZ 300SD
  • TURBO
  • 2WD
  • AUTOMATIC
  • 250,000 MILES
I cant find the sequence for what caliber to flush first?
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Friday, April 3rd, 2015 AT 8:06 PM

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Tiny
CARADIODOC
  • EXPERT
The wheel you're standing closest to.

If you're just doing a maintenance flush, which all manufacturers recommend but no one actually does, just loosen the cap on the reservoir, then open the bleeder screws and let the system "gravity-bleed". Be sure the master cylinder doesn't run empty. If it does, and you catch it quickly enough, you can refill it, then stroke the brake pedal slowly a few inches to get the air out without it going all the way down to the wheels. I can describe that in more detail if necessary.

Gravity-bleeding this way works well when you're replacing or rebuilding calipers and wheel cylinders too. You should not need a helper to do "pedal-bleeding". If you do bleed the system that way, it is very important the pedal never be pushed more the halfway to the floor. Crud and corrosion build up in the lower halves of the bores in the master cylinder where the pistons don't normally travel. Pushing the brake pedal over halfway runs the rubber lip seals over that crud and can rip them. That will result in a slowly-sinking brake pedal when under steady foot pressure, and it may not show up for a few days.

There is no specific order you have to use when bleeding or flushing the hydraulic system. If you're bleeding one wheel after repairing a leak, you bleed just that one wheel, regardless which one it is. When you're just flushing out the old moisture-laden fluid and putting in new fluid, nothing is taking place with any of the valves in the system. When brake fluid runs through the lines, it doesn't make any difference if fluid has already run through one or two of the other circuits. Some people like to do the right rear wheel first because that gets the largest percentage of old fluid, (or air), out of the system with the first wheel. Some people like to do the left front wheel first because it gets one circuit totally finished the fastest.

Be aware too that some text books and service manuals say to suck all the old brake fluid out of the reservoir with a tool like a turkey baster. The reason for that is simply to save few minutes by not having to wait for that fluid to run through the system. This job is going to take an hour, so what difference does a few minutes make? More importantly, you will never find a turkey baster in a professional shop. You're flushing the old brake fluid out to remove the moisture that normally gets drawn in. That leads to corrosion of metal parts and lower boiling point which leads to one form of brake fade. What is real serious though is brake fluid that gets contaminated with a petroleum product like engine oil, power steering fluid, transmission fluid, penetrating oil, and axle grease. If a turkey baster were used in a repair shop, the mechanic would have no way of knowing where it had been laying or what chemicals it could have come in contact with. No professional would risk sticking anything into the brake fluid that might contaminate it. Experienced brake specialists even wash their hands with soap and water to avoid getting fingerprint grease on rubber parts that contact brake fluid.

The slightest contamination with a petroleum product in the brake fluid requires the replacement of every part in the hydraulic system that contacts that fluid. That can make the repair cost more than the value of the car. The rubber parts in brake systems are not compatible with petroleum products. That will make them swell up and get mushy. Calipers will stick, and the lip seals in the master cylinder will grow past the fluid return ports and block them. Brake fluid won't be able to return to the reservoir. That trapped fluid will heat up and expand from the dragging disc brakes, and that will increase pressure and make them apply even harder. Contaminated fluid is a common cause of dragging brakes.
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Saturday, April 4th, 2015 AT 6:08 PM

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