Found a leak at a tube but don't know what it's used for

[tko]

Our pool has been losing water these past few weeks (28000 gallon in ground pool, water goes down 1/4" in 24 hours when pump is running).
I did a bucket test to rule out evaporation (it's been sunny AND windy), and there's definitely a leak.
We think we've traced it to a bad washer in the main drain connection as the bottom of the pipe feels wet (first pic, red arrow).
The other leaky spot is in the second pic (red arrow), where this black line connects to the bottom of the pipe going into the ground from the filter (I assume it drains out to where we would vacuum to waste).
The third pic shows where the top of that tube connects into the pipe going between the filter and the pump.

Any idea what this tube is for? We are thinking of eliminating that black tube and those connections altogether, but don't know if it's for pressure relief or something like that.
Also, I would describe the leaks as small dribbles. Does it make sense that we'd be losing that much water from dribbles? My fear is that there's some leak under the concrete, though the pump primes perfectly.

Thank you in advance for any insight and advice!

 

[1poolman1]

Our pool has been losing water these past few weeks (28000 gallon in ground pool, water goes down 1/4" in 24 hours when pump is running).
I did a bucket test to rule out evaporation (it's been sunny AND windy), and there's definitely a leak.
We think we've traced it to a bad washer in the main drain connection as the bottom of the pipe feels wet (first pic, red arrow).
The other leaky spot is in the second pic (red arrow), where this black line connects to the bottom of the pipe going into the ground from the filter (I assume it drains out to where we would vacuum to waste).
The third pic shows where the top of that tube connects into the pipe going between the filter and the pump.

Any idea what this tube is for? We are thinking of eliminating that black tube and those connections altogether, but don't know if it's for pressure relief or something like that.
Also, I would describe the leaks as small dribbles. Does it make sense that we'd be losing that much water from dribbles? My fear is that there's some leak under the concrete, though the pump primes perfectly.

Thank you in advance for any insight and advice!

At some point someone got wise and removed an off-line chlorine tablet feeder. That's what that tubing is/was for. It can be removed and the plumbing repaired.

 

[tko]

Thank you so much! I know you can only speculate, but would this kind of "dribble" cause noticeable water loss over a couple of days?