3900 pump won't stop pumping

3900 pump constantly pumps even when I am not spraying....Did a search and only found advise on when the pump doesn't pump...IE tap the bottom.....Any help is appreciated

i am no expert, and i am sure someone will have another answer shortly, but my 4900 did that exactly....and i was baffled, not long after it began doing that, i had a pretty big clog(come to find out) in the system and then no spray(but kept pumping still) ...i unclogged and was back to normal....the clog i had was at the base of the pump (where the big hose from the paint container connects to the pump) hope this helps....

My 3900 has done the same thing in the past. I have found that the majority of the time, there is a clog in the lower end of the pump. Remove the suction hose, and make sure it is clear. Then, remove the lower pump "bell" housing and check for a clog. I use a large pair of channel lock pliers and turn counterclockwise to remove. One more thing to look for and be careful of is the ball bearing in the bottom of the "bell" housing. Do not loose it, and take note of how the bearing is asembled into the "bell" housing. Usually, it's a simple repair. If you find no clogs, you could have a prime valve not switching from prime to paint mode.

Sounds to me that your pressure relief valve is probably bad. The original one on my LL3900II lasted a good four years. The replacement maybe nine months. I'm baffled! The repair guy says, that I probably got ahold of some trashy paint. As in, it's not his fault. Back your pressure dial back to about 60-70 percent and see if it will hold pressure. That's what I've been doing to get by until I can get it fixed. I hardly ever run at maximum pressure anyway, unless I'm using a larger orface tip (627) or in colder weather as the paint thickens. Good luck.

It's easy to check if your pressure relief valve is bad. Paint will be coming out the return tube with the valve closed. If that's not the problem, then the pump is starving for paint. You either have a clog in the suction tube, or the inlet valve is not sealing.

Every time ours has done this it has been a clog in the filter. Flush out the entire system after cleaning or replacing the filter screen, and you should be fine.