Wow, well that is seriously concerning. I am not sure the technical terms of the equipment...but I have the pump where you can set it to Filter/Backwash/Rinse/Waste and you can set it to Waste then turn the other pump off so that it sucks from behind the liner (my husband says its a 3 way valve that either sucks from the bottom or the skimmer, or turned off and nothing gets to the pump). I had a suspicion my original liner was not installed properly or maybe has a tiny undetectable leak somewhere. Thanks everyone, I definitely need to do some more investigation.
Trying to say this in a nice way, but that is not how the valve works. A very quick explanation would be this:
100% of the time the pump will suck in water from the pool (inside the pool) and never can suck in water from behind the liner.
Water will come into your pump from the bottom (main drain) or the skimmer.
The main drain only draws from inside the liner.
In filter, water is drawn into the pump, pushed through the filter, and then back into the pool. Doing this will have effect on the amount of water in your pool. This is the normal operation of a pool pump/filter.
In backwash, water is drawn into the pump, pushed backwards through the filter, and then goes out to waste (outside of the pool to a drain or creek, or your yard). Doing this decreases the volume of water in your pool. Typically done if your filter has gotten too dirty and your pressure has risen too much.
In Rinse, water is drawn into the pump, bypasses the filter, just passes through the pipes at the filter, and then goes out to waste (outside of the pool to a drain or creek, or your yard). Doing this decreases the volume of water in your pool. This should be done for 30 seconds or so after backwashing your pool to make sure teh debris from your filter does not go back into you pool.
In Waste, water is drawn into the pump, bypasses everything and goes straight to waste. Doing this decreases the volume of water in your pool. Typically done if your pool water level is to high and you need to lower the level.
So there is no way to use the pump to get water from behind the liner. If there is water behind your liner that you can detect, that is not normal and you sadly have a problem.
Now when it rains a lot, unless your pool has an overflow valve, it is normal that people have to use the pump to lower the water level.
Normal water level for a pool is the midway height line of your skimmer box. The water level should never be allowed to go under teh skimmer (very low) and never be allows to completely put the skimmer underwater (very high)