Q: Why would I have an intermittent spongy brake pedal on my 1999 Chevrolet Suburban

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Once in a while my brake pedal feels spongy and seems like it goes down further when applying brakes to stop. In the past 6 months I've replaced the master cylinder, pads and shoes, caliper on front passenger side, and bleeder screw on the drivers rear side. We bled the lines like crazy! Twice! Upon inspection the other parts are good still (i.e. roters). But I've noticed the spongy brakes every once in a while, like about 3-4 times a week (it's a daily driver). The first time the pedal went all the way to the floor before it would stop. Kind of scary! I checked the fluid level in the master cylinder and have had to add brake fluid twice since the master cylinder was replaced (about 4 months ago). I have not seen any visual fluid leaking anywhere. Could it be the power booster?

My car has 236000 miles.
My car has an automatic transmission.

Hello. Most of the time this is caused by the master cylinder failing internally. If you have replaced it with a new one then that should not be the issue. If you used a rebuilt master cylinder then that may be the problem. If the master cylinder is known to be good and if all of the brakes are in good shape then I would make sure that the rear brakes are fully adjusted. If they are out of adjustment then this will occur. If that is not the issue then there is either a failure in the ABS module or the module has air stuck in it. It is not an easy task to bleed out the ABS module as you will need a computer scan tool that has the ability to put it into bleed mode to be able to do this. If you need to have this looked into, consider YourMechanic, as a certified mechanic can come to you to [diagnose why you have spongy brakes] https://www.yourmechanic.com/services/brake-pedal-is-spongy-inspection

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