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Brakes always need to be bled after changing rear plate and rotor. If a brake slowly, the pedal feel like there is no brake fluid. If I brake quickly, it's fine but not for long. I need to bleed my brakes every 5 days, but only rear left calippper makes bubble. I can't find any leak. What can cause this?

Chevrolet Optra5 2004

Edit: I was told it is the master cylinder. Is it possible? How can I test it?

Solved: Thanks David Lively, it was the master cylinder. The fluid of the left rear and right front caliper was bleeding slower than the other two. One of the two lines of the master cylinder almost didn't flow. Changing the master cylinder solved the problem.

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If the master cylinder is leaking internally, some fluid pushes past the piston rather than pushing fluid through the brake lines (and then to the calipers).

If you're not losing fluid, I'd suggest the brake master cylinder. Easiest way to test it is to start the car and press firmly - but not quickly - on the pedal, and hold pressure. If it's bad, the pedal will gradually go to the floor.

(Note that this could also be caused by a leak elsewhere, but you'd be able to tell by fluid present on the calipers, ground or brake lines.)

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  • There is no leak on the master cylinder. The pedal doesn't hold pressure if I test as you said, but just the two or three first times and never while driving. I will buy a used master cylinder next weekend.
    – Bradford1138
    Commented Oct 2, 2017 at 15:41
  • There is no reliable way to detect a master cylinder leak on a vehicle like yours. The brakes are boosted by a vacuum system in the car and the brake fluid just gets pulled into the engine and burned.
    – Eric Urban
    Commented Oct 3, 2017 at 21:23
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Is the brake fluid going level going down in total? You basically pump up the brakes and then apply them which is what I think is going on for you, and if the total level of brake fluid has dropped then you have a leak in not the form of a connection but maybe a busted brake line running to the back brakes(on the straight section of line without connections) I might be wrong but I hope this can help you somewhat, on an impala i saw the same issues occurring and it turned out to be a hole in the actual brake line due to just wear

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  • When you bleed the brakes you release fluid also, and the frequent bleeding could be the actual loss of fluid in your case
    – sjfklsdafjks
    Commented Sep 26, 2017 at 14:33

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