Backwashing, YES backwashing

[sparkwatercleaner]

my pool is now 5 months old, i've learnt how to keep water clean to the sparkling point.
now, the pool builder told me not to pay attention to the pressure gauge for backwashing, just to look if the skimmer got enough sucktion.
ok, the pressure gauge does not change much, the sucktion in the skimmer drops a bit and I decide to backwash although the gauge reads a pressure in a green area, meaning its ok.
but if i don't backwash all the leaves will go to the bottom of the pool
HERE IS THE IMPORTANT THING
i've backwashed for 6 times more or less, but each time i do so sand comes out from the waste line (not as much as the first time but a considerable amount). i called the builder and he told me it is more the dirt that comes out than the sand, so it doesn't matter. but i DO matter and i ask myself "won't the filter be left without any sand in a year or more?" how do I realize if it's not empty?
ANY HELP WILL BE APPRECIATED

[Backglass]

Sand should not come out your waste port in large amounts during a backwash...ever. Just dirt (green/brown water). You may have a broken lateral or some other internal component that is allowing the sand to exit your filter when it shouldn't.

You can check the sand level by taking the top off and looking down inside. It should be about 2/3 to 3/4 full normally.

You should always use the pressure gauge...thats why it exists. After backwashing, make a note of the pressure when clean. Then when the pressure is 10psi or more above that number, it's time to clean out the gunk by backwashing until the water runs clear. Then rinse for a minute or two before going back to filter.

Your builder advise is like saying "Don't pay attention to the speedometer. Just look out the window and see if your going fast enough".

[Backglass]

Wait...I just realized you are a new pool owner. Are you sure it IS a sand filter and not a DE filter?

[sparkwatercleaner]

YES it's a sand filter.
The only problem I have is that the filter has a top-mounted multiport valve, and it would be difficult to take it out without having to cut the pipes!
also, the gauge has a green zone, a yellow one and a red one, meaning OK ; consider backwashing; BACKWASH!. But when the arrow is pointing the middle part of the green zone, the skimmer almost stops sucking with everything else shut. AND IT's THE ONLY ONE I have!!!

[Backglass]

YES it's a sand filter.
The only problem I have is that the filter has a top-mounted multiport valve, and it would be difficult to take it out without having to cut the pipes!
also, the gauge has a green zone, a yellow one and a red one, meaning OK ; consider backwashing; BACKWASH!. But when the arrow is pointing the middle part of the green zone, the skimmer almost stops sucking with everything else shut. AND IT's THE ONLY ONE I have!!!

Then it sounds like you have no choice but to backwash often. Sand should still not come out of your waste port. You will have to cut the pipes anyway when your filter is empty to replace the broken internal parts.