According to Honda, if you’re troubleshooting for an A/C leak and can’t find it even when using dye or another leak detector, the problem may be at the bulkhead, where the A/C lines attach to the expansion valve.
There’s a pretty good chance that the sealing surface or the O-rings have been contaminated from fiber strands that make up the fabric pad used for firewall insulation. This location makes it difficult to detect any leaking dye. Anything that leaks out gets soaked up by the insulation.
Disassemble the A/C lines at the bulkhead and check the surrounding foam padding to see if it has soaked up any dye or lubricants. If this is the case, fiber strands are most likely caught in the O-rings or stuck in the surrounding sealing area.
Replace the O-rings and remove any fiber strands from around the sealing area.