I have noticed some older Mercedes, from the 50's, have a fuel filter that has a bypass valve built into the side of it. Is the function of this valve to shut off the flow of gasoline while changing the fuel filter element?

On which cars would this be found and where? In the engine area on the left side?

An alternative would be to install inline valves before and after the filter housing, but if I could install a factory unit with the shut off valve that would be the way I would want to go.

So, do I have the function of this thing figured out or is there more to it?

If what you are describing is something like what 190 SLs' have the selector valve that is part of the fuel filter assembly selects between Reserve and Non-Reserve fuel lines from the fuel tank. There may be a position of the valve body between those two choices that blocks all fuel flow but I'm not certain of that. A push-Pull rod operates this valve from the dash knob.

Expect to pay at least a couple of hundred for a decent used one, new from MB, three times that if any are available

I have seen these reserve tank valves on VW's of that era. You can reach them from the front seat. I thought they were on Pontons during some years but they are only on 113's? I can see where Mercedes would put in a dash knob device to operate them since on the VW they were just sort of hanging there.

Perhaps the up and down stream inline valve solution is the way to go.

I know the ponton 220S has one, because that's what I have. I think that the six cylinder 219 also had the reserve lever, but the four cylinder pontons did not, but they had a low fuel warning light in the cluster instead.

__________________
Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles

yes, it is a selector for the reserve fuel line. 190SL, 220S (2 and 4 door), and probably the 219. The main issue with these is that if the seals in them are not air tight, your mechanical fuel pump will struggle to deliver adequate amounts of fuel. Another issue compared to modern filters is that the casing for the fuel filter assembly is not transparent, so you never really know if the fuel is primed or if the filter is dirty.

When I was young....

Back in the sixties, Dad had a 59 Karmann-Ghia.
It had an "L" shaped lever under the dash that you could turn for either normal or reserve feed.

Dad would leave it in normal, and when the engine stuttered, he would reach under the dash and flip it to reserve.

Why, you ask?

The car did not have a gas gage. Really.
No radio either. But it did have a big clock.

Jim

BTW, my 190SL also had the push-pull knob under the dash for selecting the normal/reserve function.

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

These reserve valves are on some 190SLs 121 chassis cars for certain.

That's how I learned about the reserve tank on VW's. I was riding with a guy in his 57 when it shuttered and he flipped the switch. The engine sprang back to life.

And why? Like you said, no gas gauge. You could get one but it was an option.

BMW motorcycles are like this (at least all the ones that I know about which are 1960s - 1993). Except the have two, because the tank has two sides that are lower than the center.

For those of you reading this in the far off future...

Earlier I asked if only the 113's had these. That was in error because it seems the 113's never had these. It was the 190SL that came before the 113 body that had them.

Asking if the 113 had them was my ignorance of the subject coming through, but as Ben Franklin is said to have said: We are all born stupid but it is our own fault if we stay that way.

As an aside, I will always remember when I first used the "reserve" feature on the 300SL. Mid 1960s, wintertime, was on my way from Boston to Middlebury, VT, in the vicinity of Greenfield, MA, when the engine began to sputter. Pulled control knob on dashboard to activate electric fuel pump which connects to fuel tank at a lower point than regular fuel pick-up allowing access to several additional liters of fuel. Got to gas station, added fuel (didn't fill tank as it has 36 gallon capacity) and continued trip. Car and electric fuel pump still running perfectly to this day.