As a non-A&P, am I allowed to fabricate and install hydraulic fluid lines as part of preventive maintenance on an aircraft?
Generally speaking someone who does not hold a mechanic's certificate may not fabricate their own hardlines or hoses, nor may they replace hardlines. This work must be done by someone holding the appropriate Airframe and/or Powerplant mechanic rating.
An owner/operator with a private pilot certificate may install prefabricated flexible fluid carrying hoses (with the specific exception of hydraulic hoses) as preventive maintenance,as allowed under FAR 43 Appendix A.
There are three notable exceptions to the above general statements:
An owner of a Special Light Sport aircraft may acquire a repairman certificate, enabling them to perform this kind of work on their aircraft.
The builder of an Experimental Amateur-Built aircraft ("homebuilt") may perform this work on aircraft they have built, as FAR 43 does not govern maintenance on aircraft with an Experimental airworthiness certificate.
If working under the supervision of an A&P mechanic you are authorized to perform any work your supervisor is authorized for (per FAR 43.3 (d), and subject to some specific requirements).