i have a 1989 gmc r3500 with a 454 that when its below 1000 rpms the power steering doesn't work but as soon as you rev it it works.its hydro boost if that helps any.the pump also runs the brakes and as far as i can tell they work fine.should i replace the pump or look elsewhere

Is the fluid foamy? Could be the pump but make sure your fluid is clean and full.

you think i should just replace pump or also replace the hydraulic brake booster

You didn't even answer his question.

sorry no its not foamy and is clean

There'* a spool valve that opens for the assist and they sometimes will get gunky and not open as they should.
Make sure the vacuum side is hooked up properly and not leaking. Check the vacuum tank and make sure it holds.
Do you have a way to check pressures?

do you think i should try a flush and new pump?i dont know if it matters but pump was a re manufactured pump.and i should have a way to check.and truck has no vac leaks.there isnt any vac lines on a hydroboost is there?

So . . . the pump currently installed is a replacement pump?

yes replaced jan 2017

Might be a bad pump or bad hydroboost. The hydroboost gets pressure first, then allows residual pressure to pass to the steering gear. Could be something gummed up in the hydroboost.

What did the original pump do when it failed?