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Spongy brakes

edited December 2011 in Repair and Maintenance
I have a 2000 subaru legacy outback with about 160,000 miles.

The brakes are spongy. The reservoir is full. I took it to my regular mechanic, and he said the lines are good. He also said that if it were the master cylinder, after pumping the brakes up the peddle will sink to the floor, which it is not doing. So he's stumped.

Does anyone know what this would be?

I've had several scares when I had to slam the brakes and wasn't sure the car would stop!
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Comments

  • When was the last time the brake fluid was changed? Brake fluid absorbs moisture from the air and that changes the brake fluid dynamics over time. Also, it allows air pockets to form within the brake system, which is what I believe is leading to the spongy feel. The air pocket will compress, the brake fluid is designed not to. Changing the brake fluid every 3 years eliminates these bad effects. If the fluid has never been changed, now would be a good time to do it.
  • When was the last time you flushed the brake system??
  • I have never changed the brake fluid or flushed the brakes.

    Will flushing the brake system cause contamination from knocking flakes off the lines?

    Thanks! Definitely something to try.
  • Change the brake fluid. You need to remove 1. moisture, 2. particulate matter and 3. air from the lines. I believe your vehicle should have had this done several times already based on the manufacturer recommended maintenance schedule.
  • It was probably changed by the dealer during scheduled maintenance, but not recently.

    I'll have it changed.

    Thanks!
  • If changing the brake fluid doesn't help then replace the rubber brake hoses.
    They can get soft and expand.
  • It is a bit scary that the mechanic didn't suggest changing the brake fluid.
    As already mentioned, that's the first thing he should suggest when someone walks in with a complaint like the OP's..
  • When the brake fluid is replaced, make sure it is done with a pressure bleeder with around 20 psi pressure. There are some places in the circuit from which the air is difficult to remove, and pumping the brake pedal or vacuum bleeding will not get those bubbles out.

    Once you are certain that there is no air in the system, the next suspect is a hose that is bulging at one of the wheels.

    Another problem that causes a symptom similar to air is a bad wheel bearing that is letting one of the rotors move a lot and press the caliper piston well back into its bore. When you apply the brakes, that piston has to go a lot farther than the other three, resulting in soft brake pedal and uneven braking. You can check for that by jacking up each corner of the car and grabbing the top of the tire and jerking in and out to see if there is any lateral movement of the wheel.
  • edited January 2012
    As others have suggested you need to do a full brake fluid flush....I use my pressure bleeder....after I suck out all the fluid in the resevoir.....I hook up my bleeder....it PUSHES.....new brake fluid down the res....and out the bleeders on the calipers.....thus replacing all the fluid.

    SOunds like you need to bleed ALL the brakes and change fluid.....IF THIS DOES NOT SOLVE IT.....It is your master cylinder...and may be all along..... the fluid swap and bleeding should be done as a good precaution and test....it surely wont hurt....like I said if after it does not firm up your pedal.....then its master cylinder time... OR as CIRCUITSMITH mentions.....the rubber lines can also be a source of a spongy pedal for CERTAIN....

    I would do this IN ORDER....
    Flush/replace fluid.....bleed all brakes......What do you get?
    Still spongy?......you can look at your rubber lines when an assistant is pushing on the brakes.....Notice any of them inflating like a Balloon? Replace any that BLOW UP.....

    After the rubber line inspection....if none are blowing up.....THEN its Master Cyl time.....not a hard job but also requires bleeding of all the calipers when finished....and DO NOT forget to properly PRIME the new master if you get to that point.....its called "Bench Bleeding" The instructions are in the box with the new master cyl

    Blackbird
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