There is a Seal that is part of the Master Cylinder that when it goes bad can leak Brake Fluid into the Brake Booster.
If you think replacing the Master Cylinder is an issue. Some People say that if the Brake Fluid leaks into the Brake Booster the Brake Fluid can ruin the Brake Booster Diaphragm.
In any event to take a look you need do what is needed to move the Master Cylinder Forward to see if there is Bake Fluid inside of the Booster.
Less of a problem is if the Reservoir Seal in the back was leaking. But, you should be able to see that.
If you have a Manual Transmission the Clutch Cylinder uses Brake Fluid from the Reservoir.
For more detailed info:
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