That would be the maximum pressure when flow is stalled (valve shut).
If accurate spec. the pump alone can never develop more than 6 psi. When you see how the pump works, it will make sense.
But now you've discovered its not the pump, but garbage coming through. So lets go there.
Take a real close look at the garbage. See what it looks like with some magnification. See what it feels like. Put some photos up if you can. Lets see try to figure out if what is coming through is the sealer, rust, or what.
Anything big should be stopped by the filter in the tank. I've seen two types of 'socks' sold. If given a choice, use the one that is cylindrical.
A paper filter will probably pass the least trash, even the smallest stuff. Chrysler used both semi-clear plastic and metal housing. So I don't worry about that.
Finally, you could install the sintered bronze filters some Holley carbs come with. Lots of guys remove them in fear of reducing flow. So maybe someone has a set in their collection they will gladly donate to your cause.
