1- Are you sure all the valves are fully open? 2- When opening the valves did yo take the precaution of opening them very, very slowly to prevent any sediment inside the piping to become dislodged and block the piping?
No pressure anywhere or only hot water faucet? I'm guessing just at the faucets. Very often when you open up the water line rust and sediment will break off and plug the screens on the faucet. Sometimes it lodges in the faucet itself. Most faucets have a screen at the end of the spout that unscrews. I'll bet that solves your problem.
The Thermostat and Temp and Press Valve is faulty and needs to be replaced. or the PRV (pressure reducing valve) is dirty and must be replaced
My guess is that the hot water heater is installed downstream from a pressure reducer to limit the maximum water pressure that the water heater is exposed to.
If you have replaced the water pump then flush the radiator with CLR also check to see if the fan is working properly.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
Check the heating elements in the water heater. These may be bad and need replaced.
More than likely it is restiction in your water heater.
Water expands when it is heated, this expansion increases the pressure. The bladder tank absorbs the extra pressure, protecting the hot water heater.
Generally this is caused by pressure being built up from the water getting hot in the hot water heater. By code you should have a hot water heater expansion tank on the cold water inlet of the water heater to absorb the added pressure. If this is your problem, the excessive pressure could possibly burst your water heater. The faucet stem or cartridge or washer may be loose or moving and blocking flow.
try switching the 2 heater hoses around, they may be backwards
Water pressure from a garden hose is sufficient.
look online or go to a handware store or plumbing store just make sure you get the right one for your brand water heater.
You would need to specify the size of the heater