I resume there is another valve for the drain too.
What happens when you turn it off?

Scott

The usual suspect is the pump lid gasket. When you shut off the valve, you are increasing the suction in the pump basket by quite a bit which is probably too much for the seal to handle.

One thing to note is that by turning off some of the skimmers, you may be increasing the main drain suction only a little bit but you will be reducing the return side pressure and flow rate by quite a bit so the in-floor cleaners will not work as well. You may be better off leaving all of the suction lines fully open so the heads have more pressure and flow to push the debris toward the main drain. The main drain does not need as much suction then to remove the debris.

An easy way to check if it is leaking air is with a hose.
Turn the system on, run water over the valve, if the air stops you have your culpret.

Scott, when I turn the main drain off or down, it doesn't create air in the pump basket.
Mas985, are you saying the in-floor cleaning heads will have more pressure and flow if I keep the skimmer on? I have an A&A system with the venturi skimmer, and they suggest keeping the skimmer suction turned down to increase the main drain suction. But will I get more water pressure coming out of the cleaning heads if I leave the skimmer suction on? Would the water out of the cleaning heads shoot further over the floor of the pool?

quote" are you saying the in-floor cleaning heads will have more pressure and flow if I keep the skimmer on? "quote
Absolutely.
A good idea of just how much is to look at the filter's NOP (normal operating pressure) with the skimmer open and then look at it with the skimmer closed.
How much did the pressure drop?
quote" will I get more water pressure coming out of the cleaning heads if I leave the skimmer suction on? Would the water out of the cleaning heads shoot further over the floor of the pool? "quote
Absolutely, good return is a direct byproduct of unrestricted suction. Poor return is a direct byproduct of poor suction (low OP)or a dirty filter (high OP)

See ya,
Kelly

I would say the main drain valve is fine and that the skimmer valve's the one with issues. Do you still have the handle or was it broken and you're using a wrench or pliers?

Scott

Great responses and all on target.

Another condition to consider would be if the main drain is blocked or undersized; it would increase the suction head creating cavitation bubbles and exacerbating oring and valve stem leaks. As Kelly said this would show up on the gauge as a pressure drop when you closed the skimmer valve.

Thanks for all the ideas. I'm doing everything each of you suggests. With the in-floor on, the pressure gauge on the filter reads about 31 psi. If I turn the skimmer off (which causes alot of air to enter the pump basket), it drops to about 29 psi. Is this enough of a drop to be concerned about? Next, I plan to run water over the valve handle and the pump lid. I'll keep you posted.

The reason that the filter pressure drops by 2 PSI is that the flow rate of the pump is reduced when the skimmer is shut off. 2 PSI is about 5 feet of head and depending on the pump, could be as much as 10 GPM difference. Not a lot but could make a difference.

It is much easier to push debris into the main drain then it is to suck it into the main drain. Reducing the flow on the skimmers is unlikely to make that much difference in the main drain performance but it is likely to make a difference in the head power.

I ran water over the valve handle and pump lid. I actually have two pump lids, one on the VS 3050 and one on the A&A Leaf Vac in which the water enters first before the VS 3050 basket. It was hard to see, but I think there may have been a difference in the bubbles when running water over the Leaf Vac lid, not so much the valve handle or VS 3050 lid. I used Magic Lube on the lids but it made no difference. I noticed the valve handle is very hard to turn, takes two hands. Other valves not so hard to turn. Could this mean something is wrong with the valve?

Quite possible. I personally hate ball valves. They score up and become hard to use, leak, handles break and people then use vice grips and channel lock pliers on the stem because they can't find a wrench or replacement handles.

Would post a picture of the equipment pad?

Scott

Scott, I don't know how to post a picture, but the only ball valves I have are on the suction side of the system. I have three suction pipes coming up out of the ground (spa drain, main drain, and skimmer lined up left to right as such) and tied into a 3 way jandy valve into one pipe going into the pump (spa on one side of jandy, main drain and skimmer on other side of jandy). All three pipes have a ball valve cut into the pipe as it comes up out of the ground. The spa drain and main drain valve gray handles are easy to turn. The skimmer handle is not easy, takes two hands. Why do I even have these ball valves? And what would make one of them almost impossible to turn?

Having valves to open and close lines is always a useful and needed item.

Ball valves on the suction side often get wacked with debris and grit, causing score marks and worn surfaces. These can and will cause the ball to rub against it's socket.

Gate valves like 2 way and 3 way Jandys are at least serviceable which extends the life expectancy under normal residential use well beyond that of ball valves.

If the above ground plumbing is also above the waterline, consider replacing the ball valves with gate valves.

Scott