I'm still experiencing low hot water pressure even after replacing the water heater. When you first turn on the faucets the pressure will be fine but within a few seconds it drops again.

Whenever you disturb the supply lines, oftentimes some debris gets caught in the strainers found at each of your sinks. Try removing and cleaning them. While they are off, turn on the faucets and flush the lines thoroughly.

If this doesn't help, there may be debris caught in your shutoff valves or they may not be turned on all the way. Pressure will build up, regardless of the size of the opening, but when flow starts, it will reduce. Have a look at the main shutoff at the water heater, it may not be on all the way.

There is definitely an obstruction somewhere.

This is an old thread, but I've got the straight scoop on this problem. I spent my weekend 2 weeks ago replacing my 50 gal water heater with a 50 gal Whirlpool/US Craftmaster. After I finally got it hooked up without leaks, I had low hot water pressure. My wife threatened to leave. After casting about these user groups for a fix, I finally came up the novel idea of calling the help number pasted on the water heater. I spoke to a very helpful native English speaker who had me probe the outlet nipple with a screwdriver. Sure enough, about 3" into the outlet nipple there was a partially occluded opening. She had me open it up with the screwdriver, and viola! I had pressure. There's a plastic insert inside the nipple that is supposed to be poked open at the factory, but they are often in too big a hurry to to it correctly. Thankfully, I had used flexible connectors instead of sweating the pipes to the unit.