If this is a W115 bodied 300D then yes there is likely supposed to be a spring in there with the filter. It has been quite a while since I changed the oil on the old '68, which I believe was at the bottom of the can and the 2 filter sections sat stacked on top of it. If this is a W1123 then I doubt that is any different from the other W123 diesel filters and won't have a spring.

Also if your pressure is down to 15 or 20 psi at idle after warm that is in the ball park. However if the lower pressures are while running, there are some problems. Post some answers back and we can probably straighten out what is going on.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150