Low Pressure FB80

We have an FB80 that we purchased refurbished. We are waiting for a tech to come have a look, but won’t be up here for a few days. At present are only getting between 5-6bars of pressure. Does not increase when blind filter is put in, nor when we try to pull a shot.
We bought a new pump and installed it, and it instantly increased to 9-10 bars, so we thought the problem was fixed, but it only lasted momentarily, and then the pressure dropped again right away.
Any ideas of troubleshooting we can do in the meantime?