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Video: The Dangers of Overfilling Your Brake Master Cylinder

When it comes time to add brake fluid to your brake system, you’ll do it at the master cylinder.

But it’s not as simple as topping off the reservoir and calling it a day.

There will usually be a “Max. Fill” line marked on the master cylinder, and it’s there for good reasons.

In today’s video, the brake brainiacs at Raybestos¬†explain why you should be mindful of not overfilling your master cylinder,¬†because it can lead problems such as:

  • Unwanted brake drag
  • Brake system overheating
  • Premature brake pad wear

It’s a good watch, even if you’re an experienced gearhead, as it delves into a bit of the hydraulic science that goes into automotive brake system design.

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7 Comments

  1. 2013 Mitsubishi outlander. both front bake areas are covered in brake fluid, both sides of the vehicle also. I probable overfilled the reservoir. My bad. pads are contaminated now. where would fluid be coming from? hoses are ok.

    • Oh no, sounds like your slave sylinders (piston) seal is worn and leaking fluid, it probably is a vehicle with more than ten years of age, you need to rebuild the caliper or even buy a new caliper. Check you piston dust seal and see if it is wet, I can guarantee you that it is wet.

  2. Sandy Whitley says:

    The brake fluid is disappearing…. After awhile the car smoked… Never did that before.

  3. PiperMarlow says:

    bosegin1995_Earn 369 euro/day, it’s here>http://moneyflow24.icu/#4ZMA9C

  4. Bill Mammosser says:

    I just did the overfill on a Ducati after replacing the caliper pistons and seals. During the bleeding procedure, I fill the reservoir as full as possible so that I can get maximum pump bleed reps. Ruh-roh! Once the brakes were bled, I forgot to set the fluid level at the proper mark on the reservoir. I went for a ride, and after a few brake applications, the fluid expanded and I had full-time brakes. I’ve been wrenching for years and should have caught this. My bad..fortunately, no damage was done. Once I set the fluid at the proper level, the world became happy again.

  5. it’s been my experience in the brake fluid is filled all the windows reservoir and the fluid expands it merely leaks out the whole or ring at the top of the brake fluid cap and spills into a a non painted area on metal pipes and down onto the ground that don’t affect anything

    I mean that’s no way to get your max Leonardo level down to maximum of course, haha, but there really shouldn’t be so much pressure from the cap that the increased pressure of the heat expansion wouldn’t cause the fluid to just leak out the top. if you have a leak somewhere else after filling it above the full Mark into the very top then you might have a broken cap a sealed cap but not to worry about nothing is really test just get a new cap can you find out where your Leach are fix that and obviously bring your break level to the maximum and not above it, and that certainly the main point break drag you can hear in the morning after the car has settled the pads will be pushed out enough by the calipers from the overfilling to wild Bill sit on the rotors put your car in neutral roll it and you’ll hear a scraping that’s normal if your reservoir is over in the film The max line, not normal if it’s at the max line or below of course.

    below the minimum line your brake light should come on when you go up or down the hill which is the best time to know if your brakes are going to fail! although I’d rather have the brake light come on could you be a better chance at life.

  6. richard wallinger says:

    yep 74 years young and did exactly the same fitted new pads all around bled the old fluid and then overfilled the brake fluid reservoir . after four miles I noticed a slight steering wheel shimmy . Pulled in for diesel and noticed the brakes were dragging / on .I didn’t apply the handbrake thinking the hot discs might overheat my new brake pads .just as I had filled up with diesel My VW Caravelle began to roll backwards. I dived in and applied the handbrake . After removing some brake fluid the brakes returned to normal . One learns something new every day ,.

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