Power Steering Help. (NOT O-ring, Pump)

My power steering light is on and the power steering is not functioning. The O-ring is replaced. And I also don't think it's the pump problem since I never hear any noise from the pump as turn. My mechanic said it's because the seal at the pressure hose. But I suspect that the seal will not lead the PS light on. Can anyone help? Thanks.

PS: Actually, the PS works well few months ago. But later, the PS will have 50% will work (If light on after car start, it won't. And vice versa) Until now, it's totally not functioning.