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Q: What causes my break fluid to leak despite having just replaced the beak lines. on 2002 Toyota Corolla

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It appears that I have a slow leak and nothing is visable under my car. I had all new break lines put in because the ones I had were rusted and had a leak.
I May 2016 I had a right front caliper and break pads put on and the rear breaks were cleaned and "tuned".
Yet I have my break light on because it's slightly low on break fluid.
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Hello,
As brake pads and shoes wear down, more brake fluid is sucked into the master cylinder to fill the void that was created due to extension of the piston in the brake caliper.

That is a mouth-full isn't it?

Simplified, brake fluid should become slowly lower over time.

If you have no issues in braking, and the brake pedal feels nice and firm (like normal), you do not have a leak.

If you have a leak, your brake pedal will begin to feel spongy, and holding light pressure on the brake pedal would cause the pedal to slowly sink to the floor.

Run by literally any shop and ask them to top up the brake fluid, or you can purchase a bottle cheaply at a parts store. The type of fluid needed is on the cap of the reservoir.
In the past week I had new break lines put in as I was told that there was a leak in them due to them being rusted.
I see nothing under the car, but the break light keeps going on because the brake fluid is low. If I don't top it off, within a week it's even lower.
Ok, I would remove the master cylinder and see if there is moisture between the master cylinder and vacuum booster. Also, if you have drum brakes, the leak may be inside the brake drums, leaking from a wheel cylinder.
Another thing, if the cap for the reservoir looks wet on the exterior, and there is fluid on the sides of the master cylinder, you may have a bad seal on the cap.
Lastly, if you remove the brake calipers and inspect behind the pads on the piston side, the rubber around the pistons may be seeping.