So where is it leaking from?
Is it from the proportioning valve? One of those lines?
Air bubble in the master cylinder could just be due to the leak as it allows air to enter the system.
So, the first step would be to fix the leaks. Then fill up with brake fluid and bleed the brakes.
I have attached the brake bleeding procedure below.
Let me know a little more information about what you are experiencing.
Images (Click to enlarge)
Monday, January 10th, 2022 AT 10:31 PM