How red does it get? Pinkish, or blood red?
Was allergy ruled out? We have a dog with seasonal allergies that sometimes shows the reaction in just one eye. It usually is fairly mild, though--just more pink than the other eye. Easy enough to check--see if your vet thinks it's okay to dose with Benadryl for a few days to see if there's any improvement.
Did the vet check for entropion? It's a condition where the edge of the eyelid turns and the lashes rub against the cornea. Seems odd that the condition wouldn't show up till age 9, though.
Maybe it's just a persistent infection that needs the antibiotics and steroids for a longer time and the meds will clear it up this time.
Hope you get some answers soon! If your vet can't figure it out, is there a veterinary ophthalmologist you can be referred to? I was surprised to hear that there was such a thing, but sometimes the specialists can pinpoint a problem that a general practice vet misses. They see so many more unusual cases than a regular vet does...