Thank you very much for that link. Somehow missed it in my google
One reason to go on about this is so that anyone else that runs into this problem will not have to do the trial and error that the folks above did and that I almost did.
So here is what I think is true.
made the 75/500 dual filter set originally with a plastic valve inthe valve body. This was in model number 15321. A valve rebuild kit of the same number was available with a plastic valve.
Due to the failures with the plastic valve the rebuild kit was replaced in the rebuild kit with a brass one which became kit number 15419.
After some time the 75/500 was redesigned and was numbered 15390.
So for those who want to rebuild their dual filters you will need to find out which valve you have. Old design number 15321 or new number 15390.
For old designs use rebuild kit RK15419 which includes a brass valve.
For new designs you may only need O rings.
I have not ordered the kit yet (later today) and will be rebuilding the valve when it arrives. I have to presume that the resistance to turning is caused by a failure of the plastic valve and that the handle sticking out further when turning is caused because the plastic valve is broken and the rough ends are moving against each other.
More later when the job is done........
Thanks again for the link!