You're bleeding the brakes after a caliper swap, the pedal comes up firm, and then it doesn't: you crack the bleeder and the fluid dribbles out instead of shooting, and the pedal sinks to the floor and stays there. Nothing is leaking at the wheels. The reservoir is full. What you've lost is the bridge — the short line or passage that carries fluid across the caliper or the axle, or the internal seal in the master cylinder that separates the pressurized circuit from the rest of the system. Once that path stops holding pressure, the pedal has nothing to push against.
What Loses Suction
The phrase gets used loosely, so it's worth pinning down. On a brake system, the thing that "loses suction" is almost always one of three components: a bridge line or crossover tube on a multi-piston caliper, a master cylinder whose primary cup has failed, or a vacuum booster or check valve that no longer holds the vacuum it needs to multiply pedal force. Each fails differently, and each sounds different when you're standing next to the car.
A bridge line failure announces itself as wetness. The line runs between the halves of a two-piece caliper, often behind the rotor or