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My 66 split window also takes a little cranking after sitting for some time, however I don't think and cannot see how a check-valve would help, as said prior, the fuel pump is a check valve, gas once enters the carb, it cannot jump out of the bowl and go back to the tank.
If a check-valve is installed it would have to go between the pump and carb.

Carb bowls are vented to the atmosphere, and given enough time, the fuel will evaporate. A check valve will not help with this issue; it will just take some cranking to get them refilled before it will run properly.

But if the fuel pump valves "leak" then all the fuel in the filter and lines can flow back to the tank, leaving a dry pump. If the valves don't seal well wet, they won't hardly seal at all when dry, so it may be very difficult to get fuel sucked up into the pump to get things running again. The solution here is to rebuild the pump and take special care to ensure the pump valve seats are smooth and clean so the valves seal well (wet or dry).

An additional check valve is just a band aid to making the fuel pump work as it should, and an additional check valve just adds more resistance and pressure drop within the fuel system. Just fix the pump and fix it right.

Well I just bought my first Pinz a K, got it real cheap as it did not run, but it had few hours on it and only 17,000 miles and looked very clean.
After doing some work on it, draining the tank and fuel line by fuel line and checking the fuel pump, because it wasn't getting fuel, however the issue turned out to be the Heater, the fuel line to the Heater or something leaked and therefore it would not suck gas up from the tank, or keep fuel in the fuel filter, I disconnected that fuel line and it pumped fuel and fired right up, it even ideals after sitting for 9 years. I did have to replace a gasket on both carbs as they were dry and rotten and leaking.

It might be worth checking if you still have the Gas Heater, as I filled the fuel filter but it would not stay there until I found the issue with the fuel line to the heater.

This problem with old fuel lines has been discussed here often in the past, but it never hurts to remind people. The heater line is often overlooked.

This problem with old fuel lines has been discussed here often in the past.

Ok, sorry if I offended anyone, but the topic was hard starting because of the fuel not staying in the fuel filter, fuel going backwards, I was letting him know where I found my problem at and that the reason my would not suck up the fuel, and why my fuel was not staying in the fuel filter, going backwards, all due to a leak in the Heater fuel line, as in my case. So if he has not found the problem yet, to try disconnecting the heater fuel line see if the fuel says in his fuel filter.

Absolutely no offense taken. This thread was a pretty old one, and I'd assume the original poster got things straightened out already. But that said, you're right, it was a similar symptom, and one which even experienced owners may be new to, as you don't generally start by looking at your heater when your car won't start.

dadnova wrote:Well I just bought my first Pinz a K, got it real cheap as it did not run, but it had few hours on it and only 17,000 miles and looked very clean.
After doing some work on it, draining the tank and fuel line by fuel line and checking the fuel pump, because it wasn't getting fuel, however the issue turned out to be the Heater, the fuel line to the Heater or something leaked and therefore it would not suck gas up from the tank, or keep fuel in the fuel filter, I disconnected that fuel line and it pumped fuel and fired right up, it even ideals after sitting for 9 years. I did have to replace a gasket on both carbs as they were dry and rotten and leaking.

It might be worth checking if you still have the Gas Heater, as I filled the fuel filter but it would not stay there until I found the issue with the fuel line to the heater.

I purchased my K not running and it had the exact same issue..