This:

Not This:Brass instruments are resonant tubes not pneumatic circuits. The essential function of their tubing is to sustain standing sound waves, not to facilitate the passage of air as if to operate cylinders and motors. Compare the length of time you can hold a note or play a phrase in one breath to exhaling freely without the instrument. This will demonstrate how little airflow there is through the instrument.

If a trumpet seems to take more effort to play, it is often described as having too much resistance. This use of the word resistance is quite appropriate: the tubing is resisting our efforts at setting up a resonance. Unfortunately, because we are correctly using the word resistance to describe this, it is often believed that the word resistance in this case also implies that there is an obstruction in the tubing or the bore is too small, making it harder to achieve a flow rate. But we know that there is no appreciable flow rate!

An instrument will present an increased resistance when the tubing does not want to resonate. This can happen when a high pressure in the wave (pressure antinode) occurs in a part of the tubing that does not want this to happen. There may be a change of hardness in the metal, a compromised solder joint, a leak, or just a bad design. A commonly found cause of high resistance in new, inexpensive beginner trumpets is excessive clearance in the valve casing, causing leaky valves.

When we look for the cause of increased resistance in a previously well playing instrument, we do not necessarily look for obstructions (although objects do get lost down the bell). Instead we look for possible leakage, especially leaky water keys or worn valves.