prostar shaking when braking

I would say its a warped drum. Out of round

 

DING DING DING I'm almost sure this is what you have, It was really hot sometime and the brakes were set and it warped it because they didn't drive a little ways so that this wouldn't happen, If you know your brakes are hot don't just stop and set them and then go home for the night, If they are hot cool them off by driving around the yard a few minutes and then park it and sit without the brakes on, unless your yard is the side of a mountain, but you know what I mean. I'm almost sure it is warped.

 

When doesn't it shake?

 

Does "slowing down" mean "braking"? Any wheels been off recently?

 

My 2010 Prostar had a badly out of round lh drum that would shake accelerating and braking. Jack up the front end and see which tire won't spin freely. If both spin very lightly apply brakes and see which one can't be turned. BTW the front drives are the only brakes that set when you set the parking brake.
Frank

 

Bad wheel bearing will give you a shimmy as well when slowing down

 

Did you ever figure out what it was

 

Out of round drums is what causes this.