Trouble with Pump

I recently had to clean up damaged Thermo/Dionix Ultimate 3000 LPG 3400SD HPLC Pump. Replaced pistons, and PM'd all seals and replaced the check valves. The issue that we are currently facing is, a fail to hold pressure. We have good check valves and no seal leaks. Yet our pressure will rise and then fall drastically, and then up again. The pump block seems to work fine, but the left most piston in the block did get rather crusty and rusted up (we cleaned this and its nice again). Any suggestions?

Equipment: Thermo Scientific - LPG 3400 SD