Need enlightenment...or is it a paranormal phenomena?

Hi, I'm new here and humbly asking for masters here for enlightenment.

My friend had just got his apartment, and he re-do the piping in the bathroom.

After about a month or so, he found that the water flow rate is going much lower than before, out of sudden. Not even enough to activate his water heater.

The water tank if full, the main valve is good, the pipes are new...He don't know what to do, I lend him a water pump and did a silly reverse flow from one of the taps to flush the water up back to the water tank.

It works, the water resumed it's flow rate and everything was good for the next few days before it happens again.

This time my friend himself went up to the ceiling, insert a flexible pipe into the water outlet inside the water tank, and blow with his mouth several times, with all the water taps switched off. Yea, this is dumb but it works!

I thought at the first time I might have flushed out some object that stuck in the pipe back into the water tank. And the object went back in so it happened again the second time.

But....this is beyond my understanding now. The water resumed high flow rate by blowing air into the outlet pipe, with the taps off.

Anyone can explain this? Please~~~~

Is this some sort of a gravity flow system where the water pumps into the tank and then gravity delivers it. A fluid cant flow from a tank unless there is a way for air to get in and take up the place the water was. Some water will flow out until there is a vacuum in the tank great enough to hold the water in.

I don’t quite understand your friends system but this is what sounds like is happening.

Welcome the site, As Bud said, we do need more on what this system is like.

If the water tank is sealed too tightly, the water cannot flow out. You may have a clogged vent in the tank. But I would like to see pictures of this system.