Dont use 3 way (or 4 way) valves in a fuel system. Such valves are
subject to 'cross port leakage' due to the contaminents in diesel fuel
oil .... sulphur + H2O = sulfonic acid which eats/etches most polished
valve faces. With cross port leakage you can't open up the 'other
side of the block/bypass without leakage. Use single valves instead;
two valves - one on EACH side of the filter housing; bring both
branches together with Tees.

In article llu7g.21455$iF3.20121@dukeread01, Glenn Ashmore
wrote:

I don't have a current picture but basically I took a pair of 500FG and
mounted them on a board. The one on the right was turned to face backwards
so both of the "in" ports faced each other. There is a three way 3/8" NPT
valve between these two ports with an SAE adapter. Center goes to the day
tank and the outputs go to the Racor "in" ports. Then on the outsides I
came from the "out" ports with an SAE to NPT elbow to a 2" nipple. Then a
NPT to Swagelok elbow and a short length of stainless tube to the second 3
way valve. The center port of this valve goes to the engine. I set the
valve handles so that they both point to the active filter.

Theoretically the procedure to change elements is (assuming you have a
couple of paper towels to catch the drips and clean up and you have either
gravity feed or a small fuel pump at the tank. )
1. Drain the crud out of the bowl of the current filter.
2. Turn both valves to the new filter,
3. Crack open the top of the filter to be changed and drain the contents
into a container.
4. Replace the old element and refill with the fuel you drained in step 3.
5. Replace the lid but leave it slightly loose.
6. Switch back to the original filter.
7. Tighten the lid as soon as you see a little fuel start to come out of the
top.

This is not the recommended procedure because Racor recommends that the
filters be in slight vacuum rather than pressure but it works. If your set
up has the engine pull the fuel so that the filters stay in slight vacuum,
refill the filter with additional fuel within about 1/8" from the top and
close the lid tightly. This small amount of air will remain at the top of
the filter and away from the out port.

All of the parts are available on eBay but the SAE fittings. They are like
hydraulic fittings but not quite and they ain't cheap.