I have a Pentair Clean and Clear 420 filter. I'm trying to clear my pool of a build up of fine silt, and this is just blowing through the filter as I vacuum from the pool. In the parts diagram, there is a part called the lower manifold, ie part number 8. It has a pvc pipe rising vertically to the top of the filter assembly, and it has a cap of some kind at the top. The cap has deteriorated and is broken. I'm not sure exactly how the filter works, but I suspect this may be where I'm getting some bypass of the filters. I may also need to replace the filters, but I want to look into this broken cap first.

Can anyone explain the purpose of this vertical pipe and the cap at the top?

The vertical pipe is the air bleed tube. It is supposed to mate up with part 5 the manifold top assembly and that then joins up with the bleed valve assembly on the tank cover. All of this is part of the system that you use to get all the air out of the filter when you are first priming the system. If it is open to the main filter tank then debris could bypass the cartridges.

Well, like I said, there's some sort of soft plastic cap on the bleed tube. It's deteriorated, and there's a hole in it. You said this tube mates up with the top manifold. It just sits loosely in a 2" hole in the center of the top manifold. Then there's a big spring surrounding that 2" hole. In the top of the cover, there's a hole where the manual bleed valve comes through.

I still don't see what this bleed tube or the spring does. If it's capped, I don't see how it bleeds air. With the broken cap, it is wide open.

I see that bleed tube listed by itself for $35-40. That's a heck of a price for a 2ft piece pvc pipe. That must be one special cap. Is there anything else I can cap that with to avoid paying $40 for a piece of pvc pipe?

You can get away with plugging the hole up, done it many of times in the past...You will just have to bleed the air out of the tank more often..

I just don't understand what this tube does. What function does this soft plastic cap serve if not blocking off the tube? Is it permeable? Is it more than just a cap, and perhaps I'm missing some parts because it's broken. What does it do that a 50 cent pvc end cap won't?

I bet this thing didn't even last two years. It's been busted for a while. I can't believe I'm going to have to spend $25-40 on this silly thing every couple of years.

We have the same issue. I have purchased a normal PVC cap and also a rubber chair leg cover in hopes of using either of these. Referring to the answer about bleeding the air out more often if we do cap it, how often would be recommended? We are kind of new to the pool world and would appreciate some assistance. Thank you so much!

The caps have a fine mesh screen that traps debris to keep the air vent clean - so you get air and not dirt. If it's not there you can get dirt/ debris that can bypass the filter cartridge and go back into the pool. The most likely break from being clogged with debris.

Check online.