Need help diagnosing a problem with my stove

Hi, i have a thelin providence pellet stove which runs great for 5-10 hours but during this time soot builds up on the glass and then pellets start to fill up and overflow. the butterfly value is open all the way, i blew out all of the dust. I'm using premium pellets. last year i put in a new fan motor and then was told the brushes were no good so i got new brushes and it worked well for the remainder of the year. I was thinking I need new brushes but im not really sure. any ideas? thank you for your help

Pellets filling up the burn pot and overflowing is a definite sign of insufficient air through the burn pot to burn the pellets. This can be caused by heavy ash blockage in the stove’s exhaust chambers or a weak exhaust fan or air leaking in from bad seals such as the door gasket.
I suggest testing the air pressure with an airflow gauge such as a manometer or the older magnahelic gauge. The common unit of measure is In-Wg and typically the reading should be less (more negative) than -0.3.
Also check the fire door seal with the dollar bill test. Close the door and see if you can pull out the bill.
Also check the damper in the air inlet to see if it is open enough to allow sufficient air to burn the pellets.

Thank you, i will try to check all of your suggestions and let you know what happens

Hi, I replaced the gasket and fan motor brushes and when i went to bed it looked great but this morning the pellets overflowed again, the damper was fully opened and the whole stove was been thoroughly cleaned out. i appreciate any help you can provide! Thank you

Hello
This is what I use im In-Wg disconnected from the flue venting to show if the problem is in the stove
@EewAAAFgGrxM5:20181117030412:s&s_kwcid=AL!2966!3!281733071183!!!g!477890585436" rel="nofollow" target="_blank">https://m.grainger.com/mobile/product/49KE06?gclid=EAIaIQobChMIzpqQ9Lfa3gIVNP_jBx3TxQ-tEAQYBCABEgIn9_D_BwE&cm_mmc=PPC:+Google+PLA&ef_id=W@@EewAAAFgGrxM5:20181117030412:s&s_kwcid=AL!2966!3!281733071183!!!g!477890585436!