Excessive backpressure is a common problem encountered in HPLC. However, there are many things that can cause a high pressure, and therefore troubleshooting may be quite puzzling. The best approach is to try to isolate where in the flow path the high pressure originates. For example, if the column is suspected, try replacing it with a union connector and see if the problem persists.
If you can trace the issue to the pump, the most likely scenario is that contaminants have built up on the PTFE frit in the purge valve assembly. This frit is easy to replace, fortunately, and all you will need is a special thin wrench for removing the pure valve assemblage from the instrument. Inside is a frit seal and a PTFE frit. This frit is designed to be disposable and hence replacements are relatively inexpensive. If you see a gradually increasing pressure over the course of your runs, the frit in the purge assembly may be responsible.