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When driving, the brake pedal is spongy and slowly goes to the floor

Tiny
CRAIGESHELBY
  • MEMBER
  • 1998 GMC C1500
  • 177,000 MILES
I have checked for leaks, the calipers and wheel cylinders are good, the pads and shoes are not worn, and I have bled the braking system more than once to no avail. The master cylinder is new as I replaced it when this started happening. Any ideas or suggestions? Could it possibly be the brake booster? Tired of wasting money and guessing at this!
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Thursday, June 13th, 2013 AT 3:35 PM

4 Replies

Tiny
CADIEMAN
  • MECHANIC
  • 3,544 POSTS
When you bleed the master cylinder do you have equal pressure from both the fittings?
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Thursday, June 13th, 2013 AT 6:54 PM
Tiny
CRAIGESHELBY
  • MEMBER
  • 2 POSTS
Yes, I bench bled the new master cylinder and had equal pressure from both fittings. But this brake problem was there before I put in the new master cylinder, therefore it has to be something else. One day my brakes were working fine, the next day they were as I described. They pump up nice and tight when bleeding the brake lines, but get spongy and go to the floor slowly when the engine is running and it sure doesn't come to stop without pumping the brake pedal.
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Thursday, June 13th, 2013 AT 7:06 PM
Tiny
CADIEMAN
  • MECHANIC
  • 3,544 POSTS
Start it up and then push the brake pedal down then bleed the two break lines. On the master cylinder. Do they have equal pressure?
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Friday, June 14th, 2013 AT 10:47 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good evening.

You need to pressure bleed the system. The van is too long to bleed with the foot method. Air may still be trapped in the system causing the pedal to go down.

https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor

https://www.2carpros.com/articles/how-to-bleed-or-flush-a-car-brake-system

To be sure it is not a caliper or cylinder, clamp off the rubber hoses with vice grips. thensee if you have a pedal. If you do, then the issue is at one of the wheels.

If it does not you have air in the system or a bad new master cylinder.

https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder

Roy
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Friday, April 17th, 2020 AT 4:41 PM

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