Delta shower faucet loses pressure when turned all the way on (hot water). What could cause this? When it first happened yesterday, a few tiny pieces of black plastic/rubber fell out of the bathtub spout. It happened when I pulled the knob up on the spout to divert the water to the shower.

DRAIN AND FLUSH WATER HEATER

Pegasus shower faucet causing nearby toilet flush to trigger tankless hot water heater to trigger. Also, all faucets in house make hammering noise when turning them off. When turning the inwall pegasus shower faucet valve water stops shut, all issues disappear. So the pegasus cartridge is at fault, though we just replaced it and the pressure regulator too. Will another cartridge/pressure regulator work instead?
Thanks

the banging noise is called water hammer and there are devices that can be added to your water lines to absorb the shock.

As for the noise, when you had the old cart out, did you flush out the faucet by turning in the water for a few seconds after removing the old one? This is a four hand job, one to control the water gushing out of the faucet and the other to conrol the shutoffs.
check seat and spring for correct assembly, the whine is caused by water pressure and flow obstruction.

It is more that likely the valve in the stem of your fixture, I'm assuming this is a single handle faucet. If you can replace the valve, you will cure your problem. I am giving you a link to how to adjust the hot water temp.

http://www.ehow.com/how_6531012_adjust-anti_scald-setting-delta-faucets.html

Check the water supply lines to the shower at the faucet handles. If the fitting for the hot water line connected to the handle is loose, it can twist and kink off the water supply further down the line when you turn the faucet handle. I'm not exactly sure how this could be relevant otherwise, but under normal design circumstances this type of scenario with your shower wouldn't matter for other faucets.

Yes, debris from water heater replacements often breaks lose and gets stuck. The problem could potentially be anywhere between water heater and the faucet. The faucets shut off valves or the Delta faucet cartridge itself. Turn the hot and cold water valves off under the sink and disconnect the water supply lines from the faucet. aim the water supply lines into a bucket and turn the valves on one at a time. If there is a significant reduction in the water pressure and volume on the hot water side, you need to replace the shut off valve. If there is no reduction in water pressure, you need to clean and/or replace the Delta faucet cartridge. Contact Delta to receive free replacement parts.

possibly the mix valve ball isn't going over to hot all the way due to the handle being plugged by lime,dismantle and clean it

Based on your description it sounds like the pressure balancing valve could be gunked up with particulate from your well or just worn out. If your water is not filtered to that shower I would lean more towards particulate. Try to remove the cartridge and take it to a plumbing parts store for a match-up since Delta makes several cartridges and you want to get it right the first time.

First try adjusting the pressure balancing gear located right behind the handle. If you still do not have hot water I suggest to replace the cartridge.

take handle off and adjust the nylon set ring it is to prevent kids from getting burned and is probably set wrong youll see teeth and a pointer just pull it out a little and turn it in the oppisite direction then push back in