Shop Vac Blows

I had the same problem with my SHOP-VAC brand vacuum. When you develop a leak in the filter bag, or filter or filter cartridge, any dust you do pick up is passed through. This tends to build up in the blower assembly and getting rid of it is difficult.

First take the hose off the machine, and take the motor assembly off of the collection can. Remove any filters on the inside filter cage. Take the motor outside, plug it in and turn it on. Dust will come out. Now while it's running take a 1 inch dowel about a foot long and bang on the sides of the blower assembly. Not hard, just enough to knock any built up dust out. Be patient, as it may take up to 10 minutes of running, tapping, running, tapping ... Turn it off and on a couple times during this operation. You will be surprised just how much built-up dust will come out.

After nothing else comes out for a minute or so of running and tapping, turn it off, un-plug the motor/blower assembly and with a damp rag, wipe down all surfaces, especially around the area that contacts the collection can.

Now put on a new filter bag/assembly/cartridge. Plug it in and turn it on again without putting it back into the collection can. Be sure you dump the collection can and with that same damp rag, wipe it out very good.

Now re-assemble your vacuum and it should work just fine. I find I have to do this about every six months or so to keep the shop vacs from spewing a great cloud of fine dust back into the shop.


from  http://community.woodmagazine.com/t5/Tools-and-Tool-Buying/My-ShopVac-Blows/td-p/5379/page/2