Basic properties of liquids and gases. Air can be drawn in where liquid can not leak.

Basic troubleshooting will point at the wheel cylinders because they have moving parts and also have seals.

Also, basic deduction will tell you to go where the trouble originated. The most common right answer to a question or problem is usually the simplest answer.

Or, as my granddad used to say, and he was one smart farmer, "That's another stone out the field, Jimmy"