I am trying to locate steps to isolate my 1994 S500's ( W140 series chasis ) rear suspension sag problem 145k miles. (I have searched the archives and am trying to locate a shop manual for sale) I would like to go through ALL the steps to determine what is REALLY causing the problem, before I just start replacing parts willy nilly.

For example how do I determine if the hydraulic pump is even working, I would think that if the pump worked and is still full of fluid the shocks must be good otherwise why isn't the fluid leaking out onto the ground near the rear end etc.?