Good brush for insert?

A lot of folks use Soot Eater with good results. I've seen the kit on Amazon for around $30.00.

The Rutland brushes are good quality.
https://www.rockfordchimneysupply.com/round-pellet-stove-flue-brush.php

Here's a couple of links to the Soot Eater found on Amazon:

http://www.amazon.com/Gardus-RCH205...d=1459536444&sr=8-1&keywords=gardus+sooteater

http://www.amazon.com/Gardus-RPS204...d=1459536444&sr=8-3&keywords=gardus+sooteater

I have a couple different ones that I use. I use a Lint Eater plastic job from the bottom to get by the cleanout tee and the short flex section. I also have a Rutland 25 Ft rod set but I had to make an adapter to fit the smaller thread on the fiber flue brush. I think the rod set was 3/8" NPT and the brush was 1/4-20 thread. I would not recommend using a steel bristle flue brush unless you can run it all the way out the other end of the flue. If the bristles are stiff, you can't reverse the direction when it's in the flue and it'll be stuck. I bought a 4" metal flue brush from McMaster-Carr and used it once. I got it stuck near the top and had to do some serious thrashing to get it out. I'm glad it wasn't way down in or I would've had to pull the flue out. The soft bristle brushes are fine for pellet stoves. The ash is light and fluffy so it cleans easy.

I personally use the soot eater to brush out my 4" PL vent (my vent pipe is about 8' in total, has one elbow and a tee) at the end of every heating season. Since your vent is 25', you'd need extra extensions for it. You connect the extension rods and use your power drill to rotate the brush. Make sure you use duct tape whenever you connect an extension rod as you don't want the rods to un-thread apart during the cleaning process. After brushing, I use the leaf blower to suck out the loose ash and spray it into the cornfield next door to my home.

You don't have to reverse direction of the drill to get the brush back out. I feed mine up from the bottom using standard right hand rotation that will only aid in tightening the threaded joins. I work the brush up and down making more forward motion than back. When it gets to the top of the 23 ft run I just keep spinning the drill in the same right hand direction and pull all the rods out till the brush is back in my hand. I have to stop to unthread each rod though because of my pipe configuration and the snaky pipe.

FWIW I also tie a piece of rag on the top rod at the brush location, I imagine that is whipping around pretty good in there. But yes, it's just dusty ash that comes down out of the vent. At least there are no chunks of creosote or carbon in mine.