I've seen road draft tubes that have never been cleaned and they still function to some extent, so it's unlikely that there's a problem with one that's been serviced, ever. Just one thing to check off before digging deeper into the problem. I like to soak a draft tube in gasoline or Diesel fuel, and set it on fire, then after it cools, tap it repeatedly with a hammer and shake the ash out of it. It's surprising how much crud comes out after doing that. Chances are the problem is in the pump itself, but you know what "assuming" makes you and me!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!