Are you sure its the flow cell that's causing the issue?
High pressure can be as a result of many things- temperature too low on Sample Manager or not activated at all, a blocked in line filter, blocked column head not to mention particulate matter embedded into any stage of the pump mechanism or injector rotary loops etc. Also, the sinker in your mobile phase container can get blocked, do you run buffered mobile phases? A faulty mixer or dirty mobile phase buffers/additives can also increase pressure. Is your flow rate constant and correct?
Try to isolate where the high pressure is coming from- disconnect the tubing from the mixer/pump valve selection right up to the detector, each stage putting typical flow through the system to check pressure. A high reading at the check mixer inlet suggests blocked check valves. Prime the lines A and B individually and check for constant flow. A long priming of about 20 minutes is often needed to clear air or blocakge from solvent lines.
Try to inject a zero volume injection and monitor pressure. If the problem is the flow cell, put all lines into first hot water (take off all sinkers and store in organic), prime up all lines for 10 mins each, then run hot water through the system (with the column disconnected, put a union it its place) for at least an hour, Repeat with Acetonitrile or Methanol, then put all lines (EXCEPT Seal Wash! Keep the seal wash line in your typical seal wash container) into a 30% Phosphoric Acid 70% HPLC Grade Water container, run at a flow of about 0.5 ml/min for at least 3-4 hours, then transfer all lines into 100% Water (again, except seal wash) and run at typical flow overnight. Check pH of eluent from waste line the next morning to ensure its pH 7. Set up system with fresh washes, mobile phase and a very long primimg/start up then try some diluent test injections. Good luck.