I have also ran the diagnosis and using the PC interface the radios seem to be working properly.

If the PC works, then it is your link from the 18Fs to the converters. Now, the PC has a 232 port I assume? How do you connect the PC to the 4490s? I would emulate this with the 18Fs, without the converters. I have worked extensively with 422s, but not 485 converters. I know the parallel resistor across A & B are crucial. If the 4490 system already has one, you may not need another in parallel on your board. Conversely, if it does not, you may need to add one.
You know the link works. The converters work hardwired, if you set up the 18Fs as quasi 232s and establish a link like the PCs (without the converters), all fingers point to the 485 converters. I used Maxims with no problems on my 422 projects FWIW.

Ron