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Second, the drain pipe might be a slow drain such that it allows some drain water to escape through it, but not enough so that the drainage tray overflows. It is very common because AC drains are slow drains that rarely get flushed out with a large volume of water, which helps prevent organic and inorganic build-up from forming. I had to use several applications of Drano foaming Pipe cleaner on my AC drain pipe recently, which eventually cleared out my AC drain that the previous owners had neglected for 30 years.
Liquid Plumber is probably the wrong product to use on an evaporator coil/drain tray, although I am not so sure it will cause more problems. The commercial cleaning solutions used to clean evaporator coils often contain muratic acid, which is a stronger, more caustic acid than the acid in Liquid Plumber. Using it a few times over the life of the coil to clean the coil should not ruin the coil or the drains. It is only when people use Liquid Plumber-type acids FREQUENTLY that people inadvertantly ruin their drain pipes.
Nonetheless, your problem on an A/C drain pipes is probably not a clog (for which Liquid Plumber or lye would be used) but a SLOW DRAIN due to build-up in the pipes, for which there are other, less caustic products designed to cling to build-up, remove it, and then harmlessly wash away (like the Drano foaming pipe cleaner).