3900 pump problem help

I recently cleaned out the elbow on my pump and replace the big blk hose and my machine was running awesome. used it for like 2 days and no it won't build pressure. the pump keeps pumping when I take the I turn knob on the side and turn it down nothing comes out the return hose? is that vavle bad? oe how do I take it off and clean it ? help a brother out Thanks

If nothing is coming out of the return hose, check that elbow again. your pump is not getting paint. Also check the hose clamps. if they loosen up that will allow air in and you will loose the prime

JPanz Thanks for the info will do I don't think the knob in the side (release vavle ?) is working. How do I remove that to clean or replace it ? Thanks again much appreciate it

Also, check the ball valve in the lower pump housing. A small piece of trash or dried paint may be (probably is) keeping it from seating. If it doesn't seat, it will not build pressure and will keep it from priming. Most likely it is NOT the prime valve. 95+% of the time when the pump won't build pressure, it is because the lower ball valve is not seating. OR Like Jpanz said, you are not getting paint into the pump. If you are not getting paint, the pump is sucking air or the ball valve is not seating. Air can also cause the paint to dry out and keep the ball from seating.

If you run the pump without pumping paint the pump packings will burn out quickly. Waterbase paints are bad about collecting on the ball valve and keeping it from closing.

Ok so where is the ball valve located ? I see someone said bottom of the pump what do I have to take off to locate it ? Thanks for the Help fellas

I do not have a 3900, but I am sure the ball will be in the piece where the hose connects to the pump.

Google Graco 3900 pump manual and you can see where it is. A picture is worth a thousand words.

Take a hammer and tap just above the elbow on the bell shaped thingy. Can't think of what it's called. Your ball might be stuck. Tapping kinda hard will break it loose.

Rock wrote on Aug 13, 2015:Go to #9

Your ball might be stuck.

Like Stripe Guy said, if you ran the pump for any length of time with no liquid flowing, good chance you need to repack the pump. Rod out the suction hose with a piece of 1/2" PVC pipe and chase with water multiple times........and check the elbow / ball check.

It would be nice if your lower ball was dirty - just unscrew the bottom piece of the pump (right above the elbow) and clean out that chamber ( that cone shaped thing). But I bet you need to replace the packings - these are the gaskets that pull paint into and push paint out of the pump. The packing kit has a schematic and directions on the back of the package. Machine will run like new when you replace these.

Thanks guys there was a bunch of trash in the cone shape "thing" below the pump cleaned it out and she ran fine. Thanks again much appreciate it

Bingo!!!! In 15+ years of striping, that has been the problem about 95% of the time for me whenever the pump will not build pressure. Second most common cause has been worn out piston packing. Glad for you that you found the problem.