I live in Africa, so my situation may be a little different than most, but the same laws of nature apply here, so maybe someone can help.
I live on the 2nd floor of a 3-story building. We have a water tank on our roof which supplies water for the building. A pipe comes out of the tank, then forms a T. To the left, water flows through a pipe down the south side of the building to all of the indoor plumbing. To the right, water flows down the west side to our two swamp coolers. There are no problems with the pipe on the left, but the pipe on the right side of that T is where there are problems. During the day, water stops flowing through that pipe. At night, it begins to flow again. I was thinking something along the lines of an air lock, but I haven't read anything that explains why it comes and goes so regularly.
I suspect it might have something to to with the temperature of the pipes. I live in a very hot climate, so during the day, the pipes (they are plastic) get very hot, especially in the afternoon (the pipes in question are on the west side of the building, exposed directly to the sun). At night, the pipes cool off a bit. Can anyone explain why this change of temperature would cause the water to stop completely during the day, then flow freely at night? Or is there another explanation? Thanks for any advice.

If not some Heat thing, then may be a valve turned off. Either electrically with timer, or photocell, or even manually.
Don't know if something in pipe that is buoyant, And when lot of water during the day, plugs up at bottom, Non use allows it to float up ? This would be strange.
I would have someone(plumber?) maybe. Shut off water, cut out 90 degree turn going into building. Turn on water momentarily to clear out any obstructions. Maybe even blow
Out from other direction also. Reconnect. Good Luck

I should have mentioned this in my initial question, but I have already checked the valves. There is no eletrical or other kind of timer hooked up to this system. It is very basic. I have narrowed the problem down to the pipes themselves, between the valve at the top, and the coolers at the bottom. Basically, there is something in the pipes, but what? Dirt? Air? Whatever it is, it only blocks the water flow during the day.

If this were my call the first thing I would want to ascertain is exactly where the cut off point is. Unless you have a electronic valve on a timer that feeds the cooler you have pressure up to the cooler in the daytime. You can disconnect the cooler feed to check and find out. This puts the problem squarely on the metering devices in the coolers. Has anyone opened them up to check?
Regards, Tom
Sorry, I was posting when your additional post came through. You have opened up the feed to the coolers to make sure that the pipes are open, or closed up, in the daytime haven't you? What valve on top? This is the first time a valve's mentioned. What does it close off. One or both feeds? Any more surprises?
If the pipe allows water to pass in the evening and not in the daytime then you may have the same problem with your pipe that we have here in Florida with our AC condensate lines. Because water's metered out of the line that means it sets in there until called for. This allows a bacteria growth like jelly to groiw inside the pipe. This jelly could expand durning the heat of the day and contract durning the nigh to allow water to pass.
The solution is to blow the line out with compressed air or high pressure water to clear the line. We then seal off the line and losd it up with bleach to kill off any bacteria that's left.
I offer this another option, let me know if it works. Good luck, Tom