Your vacuum tool still applies pressure. It just happens to be negative pressure instead of positive pressure. Either way, I think the key is not to apply too much in either direction. The -1.9 relative PSI your vacuum tool applies seems very low and unlikely to harm the seals. Power bleeders are often pumped to 10-15 PSI, and I would bet that 5 PSI could compromise internal seals in the system. I'm not sure you could use a power bleeder at a low enough PSI and still be effective.
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