I was helping a friend winterize the inground pool at a house he
recently bought for the first time. The pool is 5 years old. I've
never winterized a pool before, but have done large boats and did a
good bit of research on how to do it correctly.

We used air fro a shop vac to blow out the lines. All 5 return lines
with eyeball fittings were slightly under water. As I understand it,
the usual procedure is to remove the eyeball part of the fitting, blow
the lines with air, then with the air continuing to flow you insert
one of the rubber stopper plugs into the pipe opening and tighten the
wing nut, sealing it off. The eyeballs on this particular pump have
a male pipe thread on the part protruding from the pool wall and a
female wring with the eyeball that screws on.

We did that, but here's the problem. There was still a fair amount of
air bubbles coming out no matter how much I tightened the plugs, which
of course I thought was due to the plug not sealing. However, upon
close inspection, the air bubbles ARE NOT coming from where the
stopper goes in the pipe. That is sealed. They are coming from
where the male threaded end of the return fitting disappears into the
cover plate that goes on the vinyl liner. Same thing is happening
with the 2 that are on the fiberglass steps. The only return that did
seal correctly was the one for the Polaris cleaner and that one worked
perfectly.

So, I'm thinking that the return fittings must be cracked, possibly
from previous failure to winterize correctly? But all 5 the same
way? Also. I don;t know exactly how these are installed, plumbed,
etc, but it would look like they aren't going to be easy to replace.
Is there something else I'm missing here? One additional factor.
There has been very fine sand showing up consistenly on the white pool
steps all season, just below the 2 returns there. About every 3 days,
it becomes clearly visible and needs to be brushed off. I have been
suspicious for some time that something was not right. The pool has
a DE filter, we have taken it apart in the past, inspected it, and it
looks fine. On the other hand, the pool doesn't seem to be losing
water beyond what I would think could be attributed to evaporation.
So, if they are leaking, the leaks must be very small.

Also, I'm thinking that from a winterizing standpoint we may be OK
because while the air bubbles may be coming from a small crack in the
fitting/pipe, if water goes back the other way, it would seem that
most or all of it is going to just seep into the ground outside the
pool? Also, it would seem the pipe/fitting would be more likely to
leak with air pressure inside tending to open any crack, rather than
from water from outside the pipe coming back inside.

Any thoughts? TIA