Power Steerin Problems

Hello everyone, i'm a fairly new Disco owner that is having problems!

About a month ago i lost my power steering and was told it was the pump that needed replacing. I replaced the old unit with a seconhand pump ( it was cheaper and all i could afford ) anyway a month later (Saturday to be precise ) it went again.

Could this just be because the secondhand pump was no good or should i be looking towards something else before replacing the pump again?

I have checked all fluid levels and looked out for leaks and it seems fine

Any help would be greatly received

Thanks

You don't say which vehicle?

Maybe it's a slipping belt?

My 300tdi leaked atf for years but never failed

Sorry its the 300 tdi and the belt is tensioned fine ( or seems to be )