[Rimrock]

Those original servoes are known to break donwn pretty quickly in this machine, so it's a quite common task to throw them out.
The 4'th valve have 2 positions, open and closed. It is supposed to be fully open when the 3 other valves are not moving, letting the oil circulate nearly without resistance through the valve block and to the tank. If manually putting the 4'th valve to closed position, and the 3 other valves in closed/center position on will hear the pump work harder as it now needs to circulate the oil to the tank via the pressure relief valve, which is set to a certain pressure in order to open. It can happen that the relief valve pressure setting is too low, and/or have a internal leakage. In this case the pump will not work any particular harder when pushing oil through it. The 4'th valve is set up with a mix on the radio towards the 3 other valves. When any of those 3 valves are moved, the 4'th valve will close.
I guess you most likely have an issue with center positioning of valves after replacement of the servoes, so would continue looking closer into that. I'm not so familiar with the Hitec vs Tower servoes, I clould not quite understand why it was needed to reverse the direction when changeing servo brand. It's new to me that some servoes runs in the oposite direction from others. But maybe that is correct. It should be easy to see when testing.