We have a one year old Central Brass kitchen faucet. Since new, the hot water valve has acted funny. When you turn it on initially it gets a good flow. But as the cold water is flushed from the supply line and hot water enters the faucet, the flow decreases dramatically as the valve heats up. The valve has to be opened a lot more (up to 1/2 turn) to compensate and get a good flow. This does not happen with the cold water valve, nor does it happen with the hot water valves of any of the other faucets in our house. I suspect there is some thermal expansion of some component going on that's causing this, but why doesn't this happen to our other faucets? Poor design? Defect?

If the faucet uses rubber washers, the rubber may be expanding enough to restrict the flow. How hot is the water ? I'd probably switch the hot + cold supply lines underneath to see if it acts the same way .

I have the same thing in a bathtub faucet.

Agree with above response - thermal change in the washer.

You might try changing the washer, or checking to see if the screw holding it is loose - sometimes a deformed washer moves more.