Air lock or faulty equipment?

[csdude55]

I have a Blue Ridge Spa, which I think is the same as Gatsby but renamed. It was here when I bought my house almost 20 years ago, but I'm fairly handy and have replaced pretty much everything in it myself.

Several months ago it developed a leak, but I didn't know it until recently. I discovered that the leak was coming from the heater, and when I took it apart I saw that the pressure switch was completely blown apart! So I put in a new pressure switch and reinstalled it.

After I got it full, I turned it on and immediately had an "FL" warning... Flow Error.

I took out the filters. Nope, still get the error.

I tried toggling the jets a few times. Nope. But I had air coming out of the jets every time, so I'm pretty sure it's air lock.

I burped the heater by loosening the lid until water came out. Nope.

I went to the jet pump and opened both bleed lines until water came out. Nope.

I loosened the intake on the jet pump until water came out, then the outtake. Nope.

I used a plunger to try to force water in and air out. Nope.

Finally, I drained it dry and then refilled it, making sure that the hose was in the filter cubby and aimed right in to the main intake. Nope.

But again, when I turn the jets on for 10 seconds and turn them off, a few seconds after I turn them off I have bubbles and gurgling from 3 jet outlets.

FWIW, the filters were in place for those months that it was empty, so there shouldn't be an issue of something big that's clogging the pipes. Plus, if there was a literal clog then I wouldn't have had so much water coming out at the heater and jet pump when I loosened them.

I'm running out of ideas here. Any other suggestions?