2 Pumps To Filter. Only 1 Shows PSI?

[TexasPool128]

I have 1 pump that runs 2 drains & multiple returns. Also have 1 pump that runs the waterfall, 2 drains & multiple returns. Both feed into the same cartridge filter housing w/4 cartridges. I cleaned my cartridge filters for the first time today (pool is only 3.5 months old & they were filthy!). I was checking to make sure all was primed & working properly afterwards. I have a PSI gauge that's on top of the cartridge filter housing. I noticed when I have the regular pump running, it shows ~12psi & if I open the air relief valve on top, water squirts out (to bleed the system). I then noticed if I have only the waterfall pump running the gauge shows zero & when I open the air relief valve on top, it's actually sucking in as opposed to blowing water out.
Why does the waterfall pump cause a vacuum & the 'normal' pump does the opposite?
Is this normal?

 

[mknauss]

Does the waterfall return run through the filter?

 

[TexasPool128]

Does the waterfall return run through the filter?

I thought for sure it did & didn't ever think to look but I just went out & looked. It, in fact, does not go into the pump! That seems odd.
That answers my question(s) but raises a new question; why doesn't the waterfall return go into the filter?
It's on the opposite side of the filter housing & there a spot on the side of the housing where it could go in but it doesn't. Is this common?

 

[mknauss]

Many times waterfalls have their own suction ports in the pool and a completely separate pump system. Waterfalls need low head high flow pumps.

 

[TexasPool128]

Thanks a ton, Marty! I appreciate your input.