Ok, I said I have trouble with this area. This discussion has helped things gel a bit for me.
1. A&P mechanics are authorized to perform minor repairs and minor alterations on their own determinism or say so. The "minor" term is pretty broad to the FAA. I can remove an engine, overhaul it completely, and put it back on the airplane and that is a minor repair. I can take the wing off my Piper Arrow, pull the skin off, replace a rib, put the skin back on, and put the wing back and that's a minor repair. If I take the easier path and replace the wing rib without removing the wing, I have to add a seam to the sheet metal and that makes it a major repair or alteration.
2. AC No: 43-18 Fabrication of Aircraft Parts by Maintenance Personnel is relevant to our interests here. It states:
This advisory circular (AC) ensures that parts fabricated during maintenance and alteration have an equivalent level of safety as those parts produced under the original design holder's production certificate. ... As required by regulations, such parts fabrication and their implementation must be accomplished "in such a manner...that the condition of the aircraft, airframe, aircraft engine, propeller, or appliance worked on will be at least equal to its original or properly altered condition." This AC is not mandatory and does not
constitute a regulation. ...
Acceptable Data. Data is acceptable to the Administrator when used within the context of maintenance, a minor repair, or an alteration if the data substantiates that the product has been returned to its original or properly altered condition. Acceptable data may establish that the fabricated part complies with applicable airworthiness standards (i.e., regulations). When acceptable data is used to substantiate that the article meets the regulatory requirements and will be returned to its original or properly altered condition, it can be considered acceptable to the Administrator.
Airworthy. ...
(1) The product must conform to its type certificate (TC) ... and
(2) The aircraft (product) must be in a condition for safe operation.
Fabrication. An act in which a part/subpart is made (fabricated) and consumed by the fabricator on the product, or part thereof, in the course of performing maintenance or alterations in accordance with approved or acceptable data, depending on the category (CAT) classification of the part being fabricated and the applicable regulations. In addition, a maintenance record
entry must be made with a description of work performed, date of completion, name of person who performed the work, and a satisfactory signature and FAA certificate number.
3. Looking at the airworthiness issue: if you are familiar with the type of data to be found in type certificates then you would know that the determination of airworthiness is mostly predicated on the second determinant, i.e. the "condition for safe operation" issue. And that is up to the mechanic performing maintenance and inspection and to the pilot performing preflight inspection. So, to a very large degree, the determination of airworthiness is at the sole discretion of the mechanic.
4. FAR Part 43 and its appendices describe what constitutes a major repair or alteration. I will not reproduce that here.
5. Conclusion: I guarantee that the type certificate for a Cessna 150 has no information as to what type of hose carries hydraulic fluid to the brake calipers. As regards the airworthiness of that hose, that is entirely the province of the mechanic. Now, can the mechanic fabricate a hose essentially identical to the original? Of course he can. He has the acceptable data in AC 43.13 and will make the determination as to the airworthiness of his repair or replacement. The real question is can he remove that expensive factory hose with the odd fittings and replace it with a standard AN fitting setup? That's an alteration. Is it a major alteration or a minor alteration? Well, that's up to the mechanic to determine. If he considers it a minor alteration, then, again, he has the acceptable data and he can go ahead and make the change and determine the airworthiness on his own.