Hi
You should wash your system with at least 90% Waters + 10% organic, in addition to apply Waters procedure for acidic cleaning, which as I remember (prepare 4 solvents: water100%, phosphoric acid 30%, Methanol100%, ACN 100%), and prepare samples vials from the same solvent,and start to make injections for every mobile phase, so you can start washing with water for 15 min and make sime sample injections using sample vials, then transfer to phosphoric solution for 15 min, then water again for 15 min, then AcN for 15 min, then Methanol for 15 min, and try also to make check valve cleaning for at least one time every month by sonicate it into water+ organic, and get a small needle and try to press into big mouth of check vlv.
For Compression C/D, it should be less than 1.8 when you get small delta psi less than 1% of total pressure
I recommend to try to make leak test for pump (check until vent valve) from consule >> select pump>> maintain>> leak test, and see if it will pass or not.