prep-HPLC low pressure problem

[Kate]

I am running a prep system for purification of laboratory samples and am having a pressure problem. The system originally developed a high pressure issue and was running at > 2800 psi and hitting the high pressure limit, halting any running method.
I changed the inlet filter having found this to be the cause of the high pressure problem but have since found a low pressure problem, the sytem runs at ~ 450 psi, also (a) retention times for prep runs have become very long as a result of the pressure and (b) the organic pump head was sounding odd on a few strokes so I changed the piston seal. This resolve the noise issue but my pressure has not recovered.
Additionally, I have noticed that the pressure jumps about a bit, e.g. 435 - 478 - 493 - 464 -478 etc

Any ideas what I should do next?

My system uses Gilson 322 pumps, prep conditions are LUNA 10 C18(2) 250 x 21.20 10u and running 80 - 95 % MeCN, aqueous mp is H2O

[DR]

Check valves.

[Kate]

sorry,
what am I looking for with the check valves? I have already had a look at the check valve for the organic pump and it appears OK to me. I do not routinely use this system and require some more information from you to determine if something is wrong

[Chris Pohl]

Kate,

The thing to look for is whether or not the check valves are seating properly. If you have replacements handy, the simplest thing to do is to pop in a new one and see if this helps. Sometimes you can identify the cause of the problem by just looking at the flow out of the check valve if you have a fluid line connected to it. If the flow is pulsating, you have a problem unless you are using a single piston pumped in which case you have to expect some pulsation. There are other possible problems, though. Perhaps the seal wasn't installed properly. Any evidence of leakage?