Back problems for years now if keep legs straight little pain but when legs bent major pain?

It sounds like : Anything short of a very erect posture alters the load across your lower back causing pain so you may want to consider an exercise regimen including core strengthening and cardiovascular conditioning followed by some stretching. If no better, then get evaluated by an orthopedic surgeon to evaluate your back

Nerve compression: This is a somewhat unusual complaint, but pain associated with knee flexion (in particular, radiating leg pain), may be a sign of a pinched/irritated lumbar nerve (also frequently called sciatica). If the symptoms are persistent/debilitating, i would certainly recommend seeing a physician for further evaluation.