High pressure fluctuation on Alliance 2695e

Hello!. I am new to HPLC troubleshooting. Our HPLC recently started showing high delta psi values ​​(about 400psi) and the primary pressure goes down and then goes up; I think this coincides with the action of the piston going forward. HPLC is 10 years old (7000 injections and 230 L of solvent pumped); no MP performed.

The problem occurred when the equipment was running at 1 ml / min of 35% MPA / 65% MPB (MPA: water / methanol, 90:10 v / v; MPB: methanol; both 0.22 micron filtered and sonicated) with a column 4.6 x 250 mm C18 coupled with a security cartridge.

For start with troubleshooting, I did wet-prime for all the lines (7.5 ml / min, 3 min) and injector with methanol, many times and run the system at 1 ml/min several minutes with column attached. Problem persisted. I attache a chromatogram for system (black trace) and primary (blue trace) pressures. System pressure was about 1200 psi, pressure ripple about 350 - 400 psi (as showed in 2695 display), primary pressure went down and then rebuilded, this repeated continuosly and coincided with pistons action noise.

- C/D ratio >1.8

- Compression check test is passed

- Degasser shows 0.5 psia and makes no strange noise. No visible leaks or clogged exhaust line for it.

-No visible leaks (tested with thermal paper) for the solvent manager system and sample manager system.

-Same pressure problem when running each solvent line with methanol, with and without solvent reservoir filter.

-By-passing the 2 micron in-line solvent filter with an union did not solved the problem.

-Same pressure problem when running with two different C18 columns with and without precolumn or security guard cartridge.

-Sonicating CV's for 15 minuntes with hot water (65 °C) and then with methanol did not solved the problem.

-The same pressure behavior is showed after CV's are exchanged between pump heads.

-Flow precission test: at 1 ml/min of methanol and pressure system of 1200 psi (column attached), pumps delivered 9 ml of solvent in 09:40 (min:sec).

I didn't tried static leak test because I am not sure if subjecting the seals to high pressure for a long time could damage them, as they are old.

Is it usual for both check valves to fail at the same time?. I think if they did, that would be why I don't see a different pressure behaviour after interchanging their position, or am i wrong?.

What could be the cause(s) for the pressure fluctuation?.

Thanks!