When your vehicle has no brake pressure your brakes either have low brake fluid or you have a leak somewhere in the brake lines, wheel cylinders, brake calipers etc.
I would first check the brake master cylinder under the hood and be sure the brakes have enough brake fluid to operate.
Also if you have worked on your brakes and have opened the brake system up to the air then you need to bleed the brakes to remove the trapped air from the brakes.
You can bleed the brakes by having someone pump the brake pedal and then you loosen the brake bleeder screw on the brake wheel cylinder or brake calipers.
You need to bleed all 4 brakes to get the air out of the system.
Also look around the vehicle for any signs of brake fluid leaking and if there are brake fluid leaks they need to be repaired before the vehicle can be driven.