There are no air leaks, the master cylinder is newer and there is enough fluid... Could it be the brake booster? How can I tell?

Was the master cylinder bled properly after installation?

What do you mean by "no brakes"? Do you mean a hard pedal, low pedal, soft pedal, poor stopping power, etc...

A brake system is not very complicated if you can describe the symptoms.

A bad brake booster will stop like a vehicle with the engine shut off, a very hard pedal and lots of effort required.

Air in the system, most common problem, will have a spongy/springy type pedal with a lack of stopping power.

A hard pedal with almost no stopping power can also indicate kinked brake hoses or lines.

Post back with some more information and you will probably get some more useful/accurate suggestions.