I did maintenance on HP1100. Now the system fails thr leak test. second plateau is negative everything else pass. Pump manual does not cover this condition. Thank you for any help.

If the pump is a binary pump (G1312A) the leak is in the B pump head. Check the outlet ball valve, piston or seal. Also check that the pump head screws are tight. If it is a quaternary or isocratic pump, the leak could be any where. Pump manuals should have a clear discussion or the leak test and what a negative plateau should mean. Before you get the wrenches out, try primimg the system with isopropanol at hi flow and pressure (3-5mL/min and 350-390 bar) for fifteen minutes and repeat the leak test. I connect an old column to the outlet of the purge valve and place the other end in a beaker to prevent damage to your column. This does a nice job of swelling the pump seals and tightening the pumphead.