So devil's advocate, I don't know lots about 3d printing, but is the interface between each layer a liability for failure? If a crack develops at some point down the road and you lose all brake fluid, it would be a sorry day. Is 3D printing the best solution to the lack of availability? I suppose a better question is, how do you ensure that a reservoir will be at least as strong as a stock injected one? Being water tight is important, but being able to stand up to nvh is another issue.
No different to any other manufacturing process. Get it wrong and you get dud results. In the case of printed parts, use a real material with the right conditions and you get high performance. PA and PP are good materials with god interlayer bonding. In general, when you see layers separating [in any printed material], that is a sign of poor temperature control during print. Too low at the print head and or too cool a bed and or allowing ambient air to cool the part too fast during print.
Summary - like ALL made parts, the quality of the outcome is a direct result of the quality of the raw materials and the process control.
3D printing has two primary streams. Mostly it is pure hobby where the colour of the part is key
and people want cheap filament. Users are not technical or patient. Yeah, I know, there are some hard core people out there printing figurines etc to very high standards - but regardless, most 3d printing is of non-functional parts. The industry knows where the money is so most 3d printers have limited capabilities and most filament sold is made in bulk, sold via multi-layer 3rd parties and does not have tech data available [ooooo shiny!]
The smaller stream is of people who are doing functional prototypes and very low volume end use parts. Filament for this market is more expensive [is actual engineering material] and machines need higher capabilities. Filament has actual data sheets and test results to recognized standardized testing. There are plenty of published test results that look at printing parameters and inter-layer bonding effect on end performance. This is where I sit
Should anybody who owns any kind of 3D printer rush out and print a reservoir in any old material? Of course not and the same is true of machined parts and welded parts. You need to know what you are doing.
As for mechanical durability - yep, no way am I doing FIAT level durability work. And nor are any of the vendors of any non-FIAT parts out there