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The other day, I had been driving for about an hour and whilst stopped at some traffic lights, I pumped the brake pedal about 15-20 times, out of boredom.

I then noticed that the pedal become very spongy and ended up going all the way to the floor. Braking was difficult, but I was able to still stop the car. I tested braking hard as well, the ABS did kick in. Just the pedal was "slow and spongy", going to the floor after the brakes "grabbed".

The following day, I took the car for a test drive in the morning, the brake pedal was back to normal and was not spongy anymore, felt quite firm.

What could this be? Does it just need a bleed? The brake fluid level is at max.

Thanks.

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  • "pumped the brake pedal about 15-20 times, out of boredom." this is the problem, to prevent it, don't do it again.
    – Moab
    Commented Jul 13, 2020 at 13:43

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Several things can cause a spongy brake pedal. If the brake fluid is old, say more than 5-7 years old, it may contain to much water. Brake fluid gradually absorbs water from the air. If enough water gets absorbed it can boil when the brake fluid gets hot. This produces air bubbles in the fluid giving the same symptoms as the system needing to be bled. Once the fluid cools the air is reabsorbed by the fluid and everything returns to normal. The issue is you can't predict when the fluid will boil and the brakes ill fail. Another cause is early stage master cylinder failure. Again it is difficult to reproduce or predict when it will fail completely.

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  • So what do you think, start with bleeding the brakes, then if the problem doesn't go away, try replace the master cylinder?
    – John
    Commented Jul 13, 2020 at 11:25
  • Have your brake fluid completely replaced. If that doesn't do it. Then diagnose master cylinder.
    – Jupiter
    Commented Jul 13, 2020 at 15:24
  • I would do a flush and refill not just a bleed. If the issue returns change the master cylinder.
    – mikes
    Commented Jul 13, 2020 at 20:23
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Modern cars have dual braking system to prevent total brake failure. In the master cylinder there are two sets of pistons and rubber seals. If one of them was not sealing properly it would cause symptoms as you described, braking would require the pedal to be pressed further and leakage passed one set of seals would give a spongy feel.

Have you checked that there is plenty of Brake fluid in the Reservoir, since a lack of fluid could also cause air to be pulled into the master cylinder.

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  • Thank you, yes there is plenty of fluid in the reservoir, nearly at max.
    – John
    Commented Jul 19, 2020 at 12:18

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