central locking woes

My 1975 [116 model] is having heart attacks with the pressurised central locking on all doors. It has a leak because it won,t hold the pressure in the lines at all,it is getting worse, now i am down to 2 doors operating only [both the front doors]
I have checked and replaced a couple of the rubber seals in the individual door units themselves but to no avail. with the engine running, there is still not enough pressure to completely lock or unlock the rear doors. I have replaced the 'one way' valves under the hood but that was a waste of time also.
Any ideas as to how to diagnose where the leak is without replacing all the door and trunk units? I,d hate to do this and then find my problem may be 1 of the lines themselves. I can install an electric system like all modern day cars, but my preference is to keep it all original
any help would be appreciated