My Stanley backpack sprayer (model 61804N) has stopped working after only a few uses. It seems fluid dripped into the pump cylinder. I took the bottom assembly apart and emptied the cylinder, but it still doesn't work. The pump arm moves now, but I'm not building up any pressure. The compression chamber (up inside the tank) is also full of fluid and I can't dump it out without disassembling it. I wonder if that could be the problem?

Something like this?

https://www.amazon.com/Winters-Economical-Pressure-Internals-Accuracy/dp/B0087UCJ6I/ref=sr_1_7?dchild=1&keywords=stanley+backpack+sprayer+pressure+gauge&qid=1633991257&sr=8-7

It looks like the sprayer is made by Chapin and badged to Stanley.

https://www.google.com/search?q=Stanley+Backpack+Professional+sprayer+61804+parts

..

Chapin makes sprayers. They have parts. Just give model #. Phone 1-800-950-4458. Located in N Y STATE. Pump rebuild kit is $25.00 Inc shipping.

Hi Chris:
Seems a bit extreme to consider using the 12 gauge. I've found that a .22 works quite well. (Humour)
Hint! Pressurized tanks must have the pressure relieved BEFORE the lid can be removed.
If the lid has been over tightened or has stuck, you'll need to get more leverage to unscrew it. Sometimes a rag wrapped around it is all it takes.
You can also try pouring hot water over the cap. Lay the tank flat so the rim of the cap gets the heat. Wrap an old towel around it first so it gets a good hot soak.
Should come off without the artillery.
Cheers

I would like the answer yo that question

Drive out to the country side and look for a ACE or TrueValue hardware store. Bring your sprayer with you. They will likely have the right part for you in stock.

The picture looks a lot like a Solo sprayer I have. I wouldn't be surprised if Solo made them for Stanley. Do a search for Solo sprayers. They have a site. My sprayer is a Solo 425

the darker Seal has come off the plunger at the bottom of the unit , easy 5 min fix , just unscrew the allen keys at the bootonm take off the cover and remove the plunger , you'll see the plunger seal still inside you can fish it out with an allen key or similar tool clip it on again and reassemble

http://www.alamia.us/Solo_Sprayer_Parts_s/814.htm?gclid=CNvasPXY1ZsCFdVL5Qod3U4hJw

The handle on the right