Major knock in rear when braking

[tdrover]

I have a 2023 Trail Boss with less then 1,000km. Even since i bought it, when decelerating (pushing brake) just before stopping, i get a huge knock like i was just rear ended. This usually happens during the first few minutes then seems to lessen on every start up. Anyone know the cause or suggestions? Thinking transmission issue but i am far from being a mechanic.

 

[Stormin08]

could be the spare shifting ?? could be pads shifting in the caliper ???

but if it is THAT noticeable, i would have the dealer look it over for something being loose...suspension or some sort of major mount