graco 3900 loosing preasure

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    my Graco 3900 is loosing presur on doing long lines.
    i use SW HOTLINE paint…..thinking of switching to SW SET FAST
    i put a splash of water to thin it a little per 5 Gallons.
    presur is set at 1900 PSI

    the machine will kick while doing long lines like it always does.
    but yesterday, it would start out fine…kicking…then it would kick quickly like i was running out of paint.
    it did this for a while…..then i went back to the truck to check this out.
    though it was a clogged or something.
    so i cleaned out the machine.
    put the paint bucket back on and it wouldnt prime.
    i finally got it going!!!

    i returned to the lines, and it would hold preasur fine for a while.
    then it would start pumping fast again….as if it was running out of paint again.

    any suggestions will be greatly appreciated.


    Hello Friend
    First…thanks for the post.
    Next…you’re right…it’s dirt.
    There’s a check ball down low on the pump. When it gets stuck…the pump will just cycle.
    Next…this trouble exists in other pumps too…to the point…there’s a red, plunger thing…like a relief plug…that we pull…to free up that ball. ( Our 3900s don’t have that…! )
    Bottom line…you’re right…it’s dirt…well…old paint.
    Here’s a thing I was told…seriously…it’s a test…or…it’s the equivalent to the “relief valve” on some simple airless sprayers…
    I was told…”rap” on the “J” tube just under the pump. That should free that ball. My first thought was; “really”…?
    I took a rubber mallet and lightly smacked the J tube. It immediately stopped cycling and built pressure. That said…you can’t be going around with a hammer in your pocket.
    Bottom line…clean the machine best you can.
    Also…I use Xylene after I use Acetone. It’s a little more aggressive when it’s cleaning. I may even let it sit a few minutes.
    Last resort…take a few parts apart. Maybe disassemble from the sprocket nut. ( I am not bad with a wrench…but it’s been too long since I looked at that particular exploded view.
    Last…Sherwin Williams carries the Graco parts. There’s a replacement J tube. I just don;t remember if that lower section has the ball in it or it’s just above. Check that out. Maybe replacing the entire thing may be best…easiest.
    Let me know.


    Thanks Dan for the heads up.
    I found that hitting the pump will get it back to building pressure by accident…..i got pissed and gave it a whack…..thing went right back to work.

    Yesterday….had a small job to do.
    Got the machine out.
    Set a bucket of HOTLINE on the machine(it was half empty and already had a splash of water in it from the week before).
    So….i got out my drill….with my stiring wand…..whipped the paint up a little….
    Turned the ball valve to prime….
    And my pressure was at 2250 PSI while priming…..never seen that before!!!
    I normally spray at 1800 to 1900 PSI….it wouldn’t even push the paint.
    so i turned the PSI up all the way….and it sprayed fine.
    Finished the job.
    As i was cleaning out the machine….i clean my machine every time i use it…every time.
    I use water.
    When the second bucket of water went in…..the pressure built back up like it normally does.
    I turned it back down to 1850 PSI.
    And it ran normal.

    What do you think happened here……i think the paint was to thick.
    Property owners wanted me to do some extra striping at this same location.
    So I returned with a bucket SET FAST.
    Splashed it with the same amount of water as i do with HOT LINE.
    Turned the ball valve to prime.
    Let it run.
    Turned the valve to build pressure.
    it went to 1900 PSI.
    Worked perfect the rest of the day.

    What do you think happened?

    Any help here is greatly appreciated.


    Piedmont Seal & Stripe


    Hey Brad
    Here’s my technical answer…who knows…!
    All is well. Let that sleeping dog alone.
    That said…the bigger pressure can be a friend. I’ve sprayed at 3300. That’s over a rough road…riding…321 tip.
    Next…I do think that a little water allows the paint to “break up” a little better too.
    Bottom line…it’s thick paint. Some “batches” may be a bit different…blah blah.
    Again…move forward…!
    The other thing is this…Set Fast may not have as many “solids” in it…it will spray a bit easier.
    Again…it’s paint…made by humans.
    Keep the machine as clean as possible. ( I never clean mine. )
    And…at the very least…you have some experience under your belt.
    The machine is a good machine. The paint is good too.
    How’s all that?

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