I have no water pressure in the shower or tub in one bathroom. All other faucets are fine. When I pull the lever to switch to showerhead from faucet there is a sound almost like a sucking or a vacuum and the pressure does not change. Can anyone help me out ?

Shut the water off to the valve and open it up. Check the inlet ports of the cartridge for any crud and clear the ports. Now turn on the water to flush out the supplies. There should be a strong stream coming out of the open valve body. Reassemble and test. Good luck, Tom