If automatic shutoff is desired then check valve is one way to go. Lots to choose from. I've added a recommendation for Plumbing Supply below. I'd still add a ball valve in addition to the check valve. They even make a combo unit which seems odd until you've had a few check valves not work completely. Yes, certainly, there are many, many cases of them working. However that doesn't mean some don't fail. I assume anyone going to this kind of trouble wants the best possible results. The other choice, and the one I prefer in such cases, is to reroute the pipes. One route is to raise the out flow pipe inside the filter up to just below the water level. The filter won't drain below that level. But there is rarely room in a filter for this and you can't drain the filter unless there's another drain which is handy for cleaning. Another route is up vertically and then right back down on the outside of the filter. Most filters can handle the extra flow restriction. The top of this upside down trap has to have a way to stop siphon which can be a tee and an open riser, like a vent in a home. Can be unsightly of course. Normally the riser is very short, a couple of inches. This home made "check valve" is cheaper and will never fail. However, plumbing fun aside, I have yet to see a water garden that needs a bio filter since the entire pond is a bio filter. So I would start with finding out if the bio filter is needed at all. But I certainly don't know this pond and most people like cool stuff attached to their pond whether or not needed. Here is a link that might be useful: Plumbing Supply

Arjo, Explain where are the check valves and where are the drain valves in relation to each other. Is your system higher at the water source and lower at the end or the other way around? What is the purpose of the check valves? What is the purpose of the drain valves? How many heads on the zone? What type of heads are they? What is your flow rate and pressure and what is the flow rate of each head? Every fitting that you add to your system uses up some pressure and restricts flow. Some type of fittings use up pressure more than others. Some fittings need a minimum pressure to operate correctly. Help us catch up to what you know about your system. Thanks and aloha

"...just trickling out...." There's your clue. That isn't "pressure" other than gravity acting on what remains in the pipes. Weedmeister is correct. Annoying because it seems to go on forever. When you replace those valves, use 1/4-turn ball-valves. If you use the el-cheapos you just removed you're going round again with frozen valves when you most need to be able to close them. The ball valves cost more but so little more there really isn't any excuse not to buck up and do it right.

Okay, then there really may be reason for me to worry. I'm pretty sure the plumber didn't even go into the adjoining bathroom to turn off the valves under the sink, he just turned off the main water supply to the house when he was installing the rough-in shower valves. He spliced into the water supply lines going to the adjoining bathroom sink from within the opened wall on the opposite side of the wall board from the sink, cutting into the supply lines before they reached the shut-off valves for the sink sticking out of the dry wall on the other side. I'm not pulling your leg in any way, I'm seriously confused about why cutting into a pipe in-line and splitting it using a T would affect the volume of available water for the faucet that was originally supplied. The onloy thing I could think of as a possible reason was lots of new pipe that are now attached to those lines but are currently ful of air, not water, because their new valve has never been turned on to purge the air from the pipes. I'll look under the sink and see if the local shutoff valves were adjusted, but I'm pretty sure not since I watched the plumber walk into the water heater closet and turn off the main water supply valve for the whole house before cutting into the supply lines. Thank you for your answers, I'll point out my issue to my GC on Mon., because if it's not an air vacuum issue, there's something else wrong with the plumbing that's now behind a lot of hardibacker, awaiting tile, and I want if fixed before the tile goes up and before they have to rip into the not-being-remodeled bath on the other side of the wall to fix it. Thanks, Orchid