I have very poor hot water pressure in bathroom shower, but good everywhere else in the house. If I drain the hot water pipes and remove the shower handle and cartridge to access the pipes, can I pour CLR or a homemade vinegar solution into the pipes to clear any mineral buildup? I've already tried all the other suggestions on this board such as replacing cartridge, flushing hot water tank etc.

No. When you remove the cartridge do you have strong hot water volume and pressure?

Nope, it's not even a quarter of the volume and pressure that the cold is.

Is there an integral stop built into the inlets on the mixing valve? Or any other valves for that matter than could be restricting the flow. I am afirly certin it is not a closing pipe diameter from calcium

Do you have low pressure ONLY in the shower or in the entire bathroom that includes this shower ?

Bob may be on to something here. If your valve has screwdriver stops,(see image) built into the valve shut off the right hand, (cold) stop and the cold water supply to the water heater. Now open up the left hand(hot) stop by unscrewing the large nut on the stop. Have some one turn on the supply to the water heater. How strong is the stream coming out of the open screwdriver stop? Let me know, Tom

There are no screwdriver or valve adjustments that I can see individually for the hot and cold. There is a limit stop screw which I have opened all the way which allows me to turn the handle all the way open. The sink in the same bathroom has great hot water.
This is a symmons temptrol valve and I already replaced the cartridge. Here is a picture, hope it helps.