I heard a loud pop, and now the brake pedal has no resistance. There are no leaks in the system. I tried a master cylinder and bleeding the brakes. Still the pedal goes down easy and comes up slow. There is no vac booster on this system. I'm not sure how it works. The vehicle is a 2003 Chevy Express van.
The point you make that there are 'no leaks' points to an internal system leak. The most common fault that you will find in this description is a brake hose fracture. The flexible brake hoses are made of an internal hose surrounded by an external hose. If the internal hose fractures, it allows the brake fluid to go between the inner and outer hoses when you press the brake. When you release the brake, the fluid returns into the inner hose - hence no extenal leak. Check each flexible hose individually with someone pumping the brake. Any 'ballooning' of any hose under pressure shows it as defective and must be replaced.