proportioning valve??

The more I read Inline’s explaination, the more confused I get.
What they call a distribution block (250 on the right), I’ve always called a proportioning valve.
Regardless.
I’ve taken several of the ones on the right apart (250). I can’t really can’t tell how that switch works.
It’s got a flush Philips head on the body. That’s all I can see.
Otherwise, there doesn’t seem to be much about them to go bad.
It is all brass except for a spring and two little washers.
They usually aren’t bad.
There is an O-ring and a cupped seal on a plunger.
If you take the cupped seal out of the body (bore) WATCH OUT.
It will expand and you will play heck getting it back in.
If anyone does have a rebuild parts source, it would be nice to know.