If there is no brake pressure at all, and the brake pedal goes to floor, then check the following:
Low brake fluid
Check the brake fluid level. Top up if necessary.
Air in braking system
Bleed the braking system to check if there is any air in the system. The smallest of air in the system can cause the pedal to have no pressure.
Master cylinder issue
Check the master cylinder for leaks or if it has internally failed. If this is the case, replace unit. it cannot be repaired.
Brake pedal linkage
It is possible that the linkage between the brake pedal and the brake booster has not been adjusted correctly. Inspect and adjust if necessary.