Persistene red eye

[coopersita]

My 9 year old lab cross has had a red eye (the other eye is fine) for the past 3 weeks or so. He doesn't seem to be bothered by it at all, and there doesn't seem to be any unusual discharge.

I took him to the vet, and he first gave him an antibiotic (eye drops), which I gave 3 times a day for a week. After a week, he didn't improve, so I took him back.

On the second visit, the vet did a full eye exam (eye pressure, checked for injuries, tear production, and others, which I can't remember). Nothing seemed to be wrong. He gave a steroid to administer along the antibiotic.

After 4 days of the steroid, he seemed finally fine. I took him to the vet again, and he told me to continue with both eye drops to complete the 7 days for the steroid.

I stopped giving him the eye drops (both) on Tuesday, and today (Friday) I noticed that the eye is red again...

If it's not an infection, eye injury, glaucoma, not enough tears, what can it be? I will call the vet again on Monday (in the mean time, I started the steroid again, since that's the only thing that seems to make a difference), but I'd like to hear other opinions.

Thanks