Internal pump valve stuck open, can be inlet or outlet side
Pressure regulator stuck/needs grease
Drawing in air on inlet side
Clogged screen on inlet side... starving
Worn out pulse hose or accumulator
Searching pump pulsating here yields a ton of troubleshooting advice.
I would start with the regulator. You can probably get a kit from your supplier. Next would be chemical gage, making sure there’s no air in the lines & making sure your chemical pump is working well. Lastly, if it still pulsating, probably need to open up the pump.
Also, you probably have a short ,open end @ one end pressure hose, which you can hook up to your mount to release any air in the system. I would run it for a couple minutes so that there is no air pockets in your line. That could be the problem as well & luckily you have fixed the problem.
Why everybody thinks that replacing o-rings on valve seals is hard? All you need is an adjustable wrench and 10 minutes of time. Knowledge-wise: lefty-loosy, righty-tighty. There's nothing else to it.
If the filter is clogged you won't get any pressure.
By shutting off the chemical injection you can check if it is causing the problem
I would check all the hoses connected to your chemical injection system and chemical meter for leaks and to make sure they're tight. Also check all the fittings they connect to, sometimes they can ware out.