Here’s a quick trick I learned a while back, I’m not sure how I cam about doing this but I credit the unknown source. Fiber washers can be a bear when turning the water on for the first time on a piping system. They’re stiff and often leak, requiring you to snug the union fitting once or more to stop the leak. I found that if I get the washers wet before assembling they soak up just enough water to form a solid seal when first tightened. This typically eliminates the need to chase down leaks.