Pump draining when not running

[Robertg]

I have a Pentair 420 system and variable flow pump, only been installed (pool+equip) 4 years. I noticed immense bubbling and gurgling when the pump was starting up during the night. I determined one of the valves was likely failed (after searching on here) and proceeded to replace the actuator in it. It is the main drain line. That has not worked, so we just leave the drain line closed/off and run all cycles with just the skimmer line. Could this be a leak? I did the smoke test and do not see the pump etc. sucking any air in... ?

[Denniswiseman]

When the pump is primed and running normally do you see bubbles in the pump pre-filter basket and is there air coming out of the returns
If so then you have a suction side air leak from skimmer to pump
Does the main drain have a valve to close it off

[Teapot1]

I determined one of the valves was likely failed (after searching on here) and proceeded to replace the actuator in it. It is the main drain line. That has not worked, so we just leave the drain line closed/off

It would seem so.
if the pad is above the water line or the seal on the middle of the filter in fact a leak anywhere it would allow the water to run back over time.

Why switch it on and off? Skimmers dont work when the system is off meaning dirt and debris is allowed to sink and then requires more robot or manual cleaning. Running the pump on a lower setting (doubt you have a flow gauge) continuously keeps the skimmers skimming and improves the water quality as slower filtration is better filtration. Don't know how coarse your filter cartridge is so maybe academic.
By example, my pool runs 24/7 turns over the water 4 times in 24 hours and water quality is excellent. I only use 65watts of electricity per hour (1560 watts in 24 hours) which is less than a standard setup uses in 2 hours so massive saving.

[Robertg]

When the pump is primed and running normally do you see bubbles in the pump pre-filter basket and is there air coming out of the returns
If so then you have a suction side air leak from skimmer to pump
Does the main drain have a valve to close it off

no bubbles when running. Tried to post a pic of the setup but cant seem to on here? Anyway, the drain line when running will have some airspace in the pump window, only when that line is valved off does the pump run perfect (so runs great from skimmer line)..

[Denniswiseman]

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[Teapot1]

Anyway, the drain line when running will have some airspace in the pump window, only when that line is valved off does the pump run perfect (so runs great from skimmer line)..

Then just run the skimmer, main drains do very little, the dirt enters from the top and anything that lessens the skimmer action like running a main drain is less effective at helping to skim the surface. Dirt, leaves etc do not make it into the main drain by themselves, they have to be manually pushed into the drain. I usually plumb main drains on a valve so they can be run as returns, this circulates heated water better is safer from an entrapment point and can lift some dirt, refloat leaves from the bottom so it can be skimmed out.
I am a retired pro and my pool doesnt have a main drain!

[Robertg]

Teapot, thanks for the suggestion, heard!
Just because i like visuals, and tried prior unsuccessfully, I have added here a pic of the set up, three valves Bottom (drain), Sweeper, skimmer.

thanks all

[Teapot1]

Nice to see radius bends rather than 90 deg knuckles!