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Using the "correct" dye should not cause blockage in the system; there are some UV dye tracer kits that come with various dye types, such as dyes for: engine oil, radiator fluid, power steering fluid, ac systems, etc. Using the wrong dye, or dye carrier, could cause the existing oil in the ac system to sludge (ask me how we know that), or mixing oil types (PAG and Ester, or PAG and mineral) can result in a like problem.
Many expansion valve "problems" are the result of moisture at the valve rather than the valve itself. A 3 hour evacuation is usually long enough provided: (a) your pump is working correctly (fresh oil, cfm), (b) you do not leave moisture or air in the system .. while thinking you got them all out
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