When you say brewhead you mean without the portafilter in place, correct?
If the water flow from the wand is fine then there probably is a blockage in the brewhead passage; if so it's most likely to be calcium and you might be able to fix the issue by descaling the machine. If this doesn't help then removing the screen, boiler screw and spring/valve assembly should allow you to inspect and clear whatever the blockage is.
If the flow from the wand is poor then you could be looking at a larger calcium buildup or a failing pump. I would still run a descale cycle first before purchasing a new pump.