Maybe this is a real simple probem with to much air inside the pump head, did you try to flush the pump with 100% IPA?
The flow has to go somewhere. Could you check your waste line on the purge valve, sometimes the flow goes into the waste through that channel. Another tricky thing is to see solvent going through the seal wash lines, if these are installed. When there is a leak through the pump seals, this is one way the solvent can take.
I recommend to block the pump at the purge valve with a blank nut and let it run with a flow of water high enough get a stable backpressure at about 200 bar. Normally, this flowrate should only be only 1-3 µL / min, but when you have a major leak it can go up to 200 µL / min or even more. Once the pressure is high enough, you can check all possible connections and flow paths for leaks.