I believe that the debris is from a muffler coming apart

I have had the rubber isolation pieces for the tank lid fall off and get sucked into the pump. They are usually about four inches long and sit on the edges of the tank.

We had the same thing happen on Montgomery/Kone pump unit. It was a non serviceable enclosed muffler in the tank that came apart internally and pushed a bunch of that rubber stuff through the valve which caused all sorts of problems. Im not sure but I thought I heard this was common problem for that series of muffler. We ended up tearing the valve apart to clean it out, removing the original muffler and replaced it was a different brand muffler. After removing the muffler you could see were half the rubber baffle material had degraded and left the muffler. Just had the same thing happen on an ITI pump unit as well, except this one crapped out springs too which could be heard clanging back and fourth through the oil line as the car ran. Good times!

It definitely is coming from that series of muffler. They came out with a write-up on it years ago. The internal rubber in the muffler is breaking up. Get rid of the muffler ASAP. you may have to add a cleanout fitting to the oil line in the pit near the jack to capture pieces of rubber that have gotten through to the jack. That's after you clean out the tank and valve. Altogether a monumental mess.

Agreed. I've seen that Crap-uation numerous times. Obsolete Frank unit. Is sell them a completely new sub. That way you can assure no future problems with a warranty. Done.