Bissell Proheat only sprays for a few seconds.

My bissell proheat will only spray water from the floor or the hose for a few seconds each time I pull the trigger. I dissasembled the unit twice and checked all the lines and the heater for clogs. I even tested the motor by taking a couple hoses lose and put the intake in a bucket of water with unit running and it pumped the water at a decent pressure. I think the unit isn't getting enough water from the tank. So, I have ordered a new tank autoload reciever fitting, in hopes that this will solve the problem. Any suggestions?