In-line diesel fuel return restrictor??

Scrounged an 18 gallon diesel fuel tank off a mate , believe it came off one of those little Mazda 3000 cabovers. He jammed the diesel engine into a HQ or HZ Holden ute a few years ago, but that's another story.
Fitted it to the Kogzbilt today and whilst sorting out the fuel supply and return lines, came across this little oddity inserted in the fuel return nylon line just before it discharges back into the tank. Obviously some sort of restrictor, a bit over 30mm in length, inner bore will take a 7/64" drill bit, outer diameter is a bit over 5/16" drill bit. Looks to have two tiny metal balls in a slide. can't make out just what it's purpose may be, unless it has something to do with acting as some sort of check valve, but you can blow air in either end.
When I was cleaning the old stale fuel out, took the sending unit out to have a look inside and there is some sort of tray affair made inside the tank itself, with an inlet pipe just above the floor level It's either the main supply line pickup, or could it be some sort of reserve?
There are actually THREE steel pipes going in/coming out of the tank, so take it that the larger (roughly 5/16" pipe) would be the main supply line, other two are bit smaller, maybe 1/4". Would say that the one that had this device in it would be the excess fuel line return, only thing I can think the third one would be is some sort of breather. The cap on the tank is a screw on type with a gasket in it, so the tank would have to be able to breathe. Must have just vented to the air , or originally had some sort of hat on it.
Had a squiz through the land Rover engine book, where the replacement engine came from, only shows TWO lines, main supply and reurn excess fuel, no mention of any sort of restrictor. Original vehicle that the engine came from, the fuel tank probably just vented through the filler cap??
Anyone throw any light on it?
Wouldn't have been too much pollution gear around when either the engine or the fuel tank were current, mate thinks the Mazda was early 80's, know the engine is 1992/93.
Cheers, Dave

Hi,

it maybe a check valve as you said but for when the vehicle is upside down in an accident to stop fuel coming out.

Terry

The valve could be to stop fuel from draining out of the return line and prevent air going back up the return, and it may also provide a restriction to maintain some sort of back pressure while the engine is running. The tray in the bottom of the tank is to keep some fuel around the pickup when the tank is almost empty, and the vehicle is cornering, etc. Of the three pipes, one will be the supply, one will be the return and the other will act as the breather while the tank is being filled. This one usually vents just under the cap threads.

The overflow usually had a non return so dirty unfiltered fuel couldnt be pulled through if the filter was allowed to plug up.

So, could be either of the above? Maybe does both jobs?
Either way, it can go back in once I get hold of new nylon lines.
Doesn't seem to act like a one way check valve (unless it's cactus, of course) and although limiting the amount of flow, the return excess fuel would only be a bit more than a dribble. Not like it's going to cause some sort of pressure build up in the return line, so can't see it harming anything.
Maybe something required by law in case of roll-over, who would know.
Thanks Guys,

Dave, if the restrictor came with the tank and not the Rover donk, I would leave it out. Some engines rely on the excess fuel returning to the tank to keep the injection pump cool so if there is any sort of a restriction, the pump might get a bit hot.

I think the excess fuel can be plenty when the engine is idling.

I'm with Terry, in that it goes on the vent hose to stop fuel coming out when parked on steep ground with a very full tank.
Steve.

Dave, I have a couple of 89-92 3500 Mazda's. The fuel lines are supply, return and vent as the tank does have a threaded cap with no vent.
The vent line does have a little one way valve which will sometimes whistle on hot days after the diesel has been shut down.

roKWiz, thanks for the advice and the photo, looks to be the same tank or close enough to it. Bloke told me tonight that you are spot on, said that the VENT line could have either gone to a canister, been vented to the air or again, been returned back to the top of the fuel filter. He also added that as a rule, the vent outlet should be higher than the injector pump, didn't really say just why though. Apparently this regulation came in as early as 1978. Said under no circumstances should any restriction be placed in the excess fuel return line, could as was noted here, do damage to the pump. Thanks to all for the invaluable help. Cheers, Dave