Brake fluid is leaking even though I cannot see the leak. The brake fluid in the master cylinder drops every few days and keeps the "brake" light on.
It is possible that your brake fluid could be leaking into the power brake booster. This is the big round container that the master cylinder is bolted to. The brake booster operates off of the vacuum created by the engine to increase the amount of pressure that can be applied to the master cylinder with your foot. If a seal has failed in the back of your master cylinder it is possible for the fluid to be sucked into the booster. This would create a non visible leak without further inspection. I would suggest you enlist the help of a mechanic to check your brake hydraulic system for leaks to confirm where the brake fluid is going.
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