Pool Pressure Drops about 5 PSI - when valve set to full Main Drain

Hello - I'm curious if anyone has any thoughts on this - I've noticed that when I set my valve to full main drain, the filter drops about 4 or 5 PSI to around 14PSI. Normal filter pressure when on full skimmer or a balance between skimmer and main drain is about 20PSI. Does this indicate a leak with the main drain pipe back to the pump ? Thank you in advance

Probably just indicates that the main drain line has more head loss than the skimmer which is not unusual at all. Probably a longer run and maybe smaller pipe.

You want most of the flow from the skimmer anyway because it removes debris from the surface. MDs don't do much at all.

Thanks for the reply - both good points It would be a single pipe as opposed to two and it's definitely a longer run.