When the system can not correct due to it being at limit , it
is best diagnosed with diagnostic equipment ..
However, a DIYer look around of all vac hoses for breaks/disconnects is where you can start...the common 104 places are the hoses running in front of the engine to the switch-over-valves and AIR pump.
Best to go to Search feature to find these locations..
Same for the fuel pressure regulator... there are several threads on that..
It also has a vac hose . If you disconnect the hose and see dripping gas..it is faulty.
Again, these are just simple possibilities that you can look for , but after that, the proper Data/Tech diagnosis is recommended
if you do not have Meters/exper. etc....