I have a single hole delta handle that has good pressure cold but hot the pressure drops and only luke warm water on maximum

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

The way the faucet cartridge is designed it will protect you from large changes in water pressure. This can happen if someone is useing the shower and someone else flushes a tolet there is a sliding valve in the cartridge that will close off hot water supply to protect you from getting scalded.If you are not getting hot enough water there are adjustment you can make to increase the maximum setting.
As long as you are getting good hot water supply in other fixtures in your house I doubt that changeing the water heater is going to help.
I think that you may have a defective cartridge and I would call delta again and give them another chance. They generally have very good customer service, and perhaps the one you talked do did not understand that you have good hot water supply elsewhere.
If you do not have hot enough water to shower valve that can eaisily adjusted by removing set screw that holds the handle on and remove the handle.If there is a white temp. limit disc on the stem then remove it. Slid the chrome sleeve from around the valve body off. You should see a disc that you can slid out a little and rotate to the left a small amount. This adjusting disc and its mating surface have teeth on them that mate. There should be marks on the valve and disc with a arrow on them to show which way to turn for hotter water.Reassemble and check.
If your water starts out hot and then gets cooler while being used and you still have hot water in other places then I think you have a defective cartridge. It should be under warranty. Hope this helps you some. Good luck and thank you.

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.

Pull apart the front of the valve and there is a limiter that is set to limit hte hot water, you need to adjust that limiter. I find many valves are shipped with teh setting too cold.

On a single lever shower handle, when the handle is pointing straight down the water is off. The handle will then rotate 270 degrees to the right through cold - warm - hot. What has happened is when you took the other handle off the new handle didn't go into the same position as the old one. So now when the handle stops at the down position the valve is still open probable running cold water. The fix is to take the handle back off. Rotate the valve into the off position so that water stops flowing and reinstall the handle pointing down. Now when you rotate the handle to the right you will get cold then warm then hot at 270 degrees and back to off when the handle is pointing back straight down. If you are still having problems please let me know and we will examine it a little closer.

A rebuild kit for a shower is pretty cheap for delta. the actual ball behind the handle maybe wore and not allowing it to seat properly and restricing water flow hot side.

Also if you take it appart the 2 washers and springs you might as well replace them also.

Replace washers and springs and the ball with shaft that handle fits to.

Shut off water before taking appart.

it can be adjust take handle off and remove locking collar first turn water off,

it has a metal or plastic ball with a shaft for the handle it may have spun or a washer may be bad under it,

you can get a replacement at lowes or Home depot to replace it if this is the problem .

That is a graduated water mixer valve, and it sometimes has a little white water restrictor blocking flow from your hot water port. See if that was left in by removing the handle with all water shut off to the valve and reattach then try again. This can also happen if you have a tankless water heater because pressure controls heat. Typically a Tankless water heater system will require little, to no cold water to your bathtub valve because of the heavy water flow at the tap. Remember, flow equals temperature. Less flow = more heat, more flow = less heat.

i have a 7570 salora that took the top off and i can't get it back together right

Sounds like you have lost pressure on one side of the valve. ( either hot or cold ) Or the cartridge has gone bad. If you have hard water if probably is the cartridge. There are a couple of different series for your style of faucet.

The reason why you only get a few drops is the cartridge is pressure balancing. It is a safety device which prevents scalding if one side of the supply shuts down.

Please rate this response and Good Luck, Phil