Here is a method I use to track down leaks in the brake system.
1. Cut a 2 X 4 so the length will wedge between the brake pedal and the seat with a good bit of pressure on the pedal. If needed clean the connections and brake lines of any brake fluid, oil, grease, etc.
2. Fill your master cylinder with fluid. Get the best high intensity light you have and a hand held mirror (to look at the back side of the line) and examine every inch of the brake lines and connections while the system is under pressure with your 2X4.
3. If you have a leak in the system this method will uncover it. If you have new fluid and a small leak it is harder to detect so you will need a really good light. I have seen some leaks that were so tiny I had to leave the system under presure for 15 minutes before they would show up.
4. If the fluid is leaking out, that also means you have air getting in, which will have to be fixed before you can blead the brakes.
5. If you suspect the master cylinder itself, try re-bleading the master cylinder seperatly first, then re-connect to the rest of the system. You can bench blead the master cylinder without taking it off the car. Don't assume if your master cylinder is new that it is good. I have seen plenty that were bad right out of the box. You will get bubbles continually when bench bleading if its bad. If you got all the bubbles out when bench bleading and then have them re-appear after you hook your lines up, you have trouble with the connections or lines going to the master cylinder.