When hearing air while applying brake could it be master cylinder Know fluid is leaking from wheels?

If there is fluid leaking in or on the wheels, your problem is most likely there at the leak. When the fluid fails to reach the wheels is when the master cylinder would be in question. Most often if you find and repair the leak, you have fixed the problem. After fixing the leak,make sure you fill your fluid to the full mark after you bleed the brakes to prevent any air from corrupting the system.