The pump is new so it's unlikely that is your issue, especially if replacing it didn't change anything. The only way to test it's pressure is to hook up a pressure gauge; I have a portafilter that I modified to include a gauge that I use for this purpose.
The most common causes of weak flow from an empty portafilter are a clogged brew screen or coffee basket ,or a dirty or malfunctioning portafilter. When running the machine with the portafilter in place, does water leak from anywhere, such as around the edges of the portafilter or from the steam wand? I would also open up the brew valve again and double-check that you installed the silicone nipple in the correct orientation, ie. facing up towards the boiler. There's very little else that can effect the brewhead pressure in these machines.