Just so we're all on the same page here... Are you referring to this thing?
And when you say "it holds the right amount of pressure:"
1. Does the Disk Indicator stay
within the black rectangle, within Maximum and Minimum flow?
2. Are you testing when the pump is half full of air? Or when the pump is primed and running fine, with only a few bubbles just below the pump's strainer cover?
I think you should refer to this as your flow gauge (which is what it is), and "indicating the right amount of flow," unless you all want to call it something else. It's not really "pressure."
For what it's worth, I've never thought to use mine at the end of the hose. I first saw my pool guy use it at my vacuum port, without the hose ('course, he was an idiot). And to be honest, I've never read my Rebel's instructions. Hang on... OK, just did (they were still sealed in plastic!!) The gauge goes at the end of the hose. I wonder if the OP could do both, and if that would tell us anything?
The wooden toy was certainly a setback. It's hard enough to troubleshoot
one problem at a time, right? But at least that one is behind you. And the notion that a suction leak might only reveal itself under increased suction is a tough one to figure out, too.
This is quite a mystery. In cases like this, I revert to my tried-n-true troubleshooting technique. Which is what we've all been suggesting of sorts, but maybe not systematically enough. You dismantle or otherwise eliminate everything you can, to get back to a "no problem" state. Then you add back
one thing at a time, as much as possible, until the problem resurfaces. That last addition is
usually the culprit. I say usually, becomes sometimes it's only a combination of things that causes the problem, but we'll cross that bridge.
I'd suggest you start over, now that you've corrected for the toy. You'll be tempted to skip steps, thinking "That can't be it." or "I've already done that." You shouldn't. And please read this suggestion in its entirety, before you start. ESPECIALLY THE LAST PARAGRAPH.
BEFORE EACH OF THESE STEPS, BE SAFE, TURN YOUR PUMP OFF, EACH AND EVERY TIME.
Take out of your pool system every component of your cleaner system.
Every component. Will the pump function correctly for 24 hours, without losing prime? Then the problem is with, or related to, the cleaner system. (Again, I know you think you know that. "Just do the work.")
Put the flow gauge into the skimmer's port (if it fits, skip this step if not). Can you get the gauge to indicate proper flow without losing prime? (Doesn't need to for this part of the test.) I need some other TFP'ers help here? How long can the OP leave that gauge like that? Will that hurt the pump? Leave it for as long as is safe for your other equipment and see if you lose prime. Hours, if you can.
Then repeat that last step, adding in one component of your cleaner system at a time, in between the gauge and the skimmer port. Seriously. ONE AT A TIME! Starting with that adjuster valve thingie you mentioned (Pentair's Regulator Valve I presume). Adjust the valve for the proper flow rate. Observe the pump. Observe the gauge, again for as long as is safe for your other equipment.
Then the first section of hose, then the next, etc. Gauge always last in line.
Then replace the gauge with the cleaner. I'd suspend the cleaner somehow to keep it from roaming around. And test that.
Then finally let the cleaner go to run normally.
Keep track of the gauge reading for each trial. Observe the pump each time. Run each trial for "as long as is safe for your other equipment."
If one of these trials results in the pump prime issue, then replace that part. See if that solves the problem. If not, come on back with all your results. If you skip steps, or combine steps, or shorten durations between steps, to speed things along, you only increase the chance of missing what is causing the problem.
WARNING: This is how I'd do it, but BEFORE you attempt this yourself, I'd highly recommend you let others chime in to see if they see a flaw in my MO. You're dealing with some very powerful, very expensive equipment. I'd want at least a few more sets of eyes on this to make sure I haven't left something important out.