It sounds like you've done some thorough troubleshooting already. Since you've confirmed vacuum at the booster and there's no air in the system, it does indeed point to a potential issue with the brake booster. Before committing to a replacement, you might want to double-check the vacuum hose for any leaks or blockages and ensure the check valve is functioning
properly. If everything appears to be in order, then replacing the booster might be the next step.
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