With the engine at high idle, open the bleed screw on the SLS valve. If the fluid is clear, the tandem pump is likely OK. If the fluid is foamy and stays foamy, pump is not ok. If foamy, disconnect the rubber hydraulic hose (high pressure output side of the TP) where it connects to the metal line (on the wheel well near the exhaust manifold) to confirm the fluid is foamy. [Aside, when I installed my rebuilt TP, I had to disconnect the hydraulic hose to get the pump to "prime"] Continuous foamy fluid means worn out TP.

If you put the rear end on ramps and move the valve to the raise position, does the rear raise? If so, doesn't that mean you have enough pressure in the system? The FSM you host has a section to test pressure on those lines as well.