Common Backpack Sprayer Problems and How to Fix Them

Backpack sprayers are an effective, comfortable way to transport any sprayable liquid. They are mostly filled with fertilizers, weed killers or pesticides, although they can also safely spray various construction chemicals, such as paint remover. While there are various models in the market, and from a number of manufacturers, these units can have a fair share of issues. The good news, however, is that many of the problems with these backpack sprayers can easily be fixed. In the following guide, we take a look at some common backpack sprayer problems and how to resolve them to help users make the best out of their units.

Why is My Backpack Sprayer Not Building Pressure?

If the sprayer does not build pressure, this may be due to several issues. To resolve the problem, try these troubleshooting tips below:

Inspect piston seal on pump

Locate the piston seal and check to determine if it is damaged and needs replacing.

Inspect the pump cylinder washer to see if it is dirty or damaged.

If dirty:

Fill the tank with clean water and flush pump to remove any debris from the pump cylinder washer.
If this does not fix the issue, you will need to replace the pump cylinder washer.

Backpack Sprayer Leaking from the Bottom

If your sprayer leaks from the bottom of the tank, here is what you should do to troubleshoot and resolve the problem:

Inspect piston seal on pump

Locate the piston seal. The seal should be periodically cleaned and recoated with a heavy coating of petroleum jelly to ensure a tight seal within the cylinder. If the piston seal appears to be worn or damaged, it will need to be replaced.

Check for damaged or worn o-ring(s)

Locate the o-rings on the pump cylinder. If the o-rings appear worn or damaged, they will need to be replaced.

If you have a Field King backpack sprayer, follow these steps to service:

Position pump handle all the way up, placing the piston in the down stroke to allow access to the screws retaining the crank arms.
Remove the pump handle. Remove the 4 screws securing the crank arm to the drive shaft using a 6mm Allen Wrench.
Next remove the screw that secures the crank arms to the drive links using a 5mm Allen Wrench.
Remove the pump cover by inserting a flat head screwdriver into the lower snap windows of the pump cylinder.
Withdraw the crank arms from the drive link through the side of the pump cylinder.
Now the drive link and piston assembly can be removed straight down through the bottom of the pump cylinder.
The piston cup and cylinder walls should be periodically cleaned and recoated with fresh petroleum jelly. Take note of any tears or scratches and replace as needed. Take care to guide the piston cup past the side openings in the pump cylinder when reinstalling.
You will need the piston seal kit to rebuild the pump cylinder assembly. First remove the pump cylinder by unscrewing it counterclockwise.
Use a Phillips head screwdriver to remove and discard the pump cylinder washers, saving the screw and stainless steel washers. Replace with new. Do not grease pump cylinder washers. Inspect o-rings for wear and replace with new and/or lubricate with petroleum jelly.
To reinstall the pump cylinder, rethread into the pressure cylinder until the large o-ring is not visible then continue turning until the tab and notch align.
To replace the pressure cylinder o-ring, remove the pressure cylinder by loosening and removing the clamp ring that retains the pressure cylinder. Use a block of wood for a punch to drive the pressure cylinder out of the bottom tank opening. Withdraw the pressure cylinder and remove the old o-ring. Clean the seal groove and mating tank surface with a soft cloth. Replace the o-ring and coat with petroleum jelly.
To reinstall pressure cylinder first warm the bottom tank opening by running it under hot tap water. This will ensure that the pressure cylinder seal is seated properly. Take care not to roll the o-ring seal out of the groove as you reinsert the pressure cylinder. Realign the tab on the tank with the notch on the pressure cylinder and make sure the flange on the pressure cylinder is flush with the flange on the tank. Reassemble the clamp ring and tighten.

Backpack Sprayer Not Pumping

If the backpack sprayer is not pumping check for an airlock in the line and pump. Air can get locked in between the tank and the trigger. To fix this problem, simply turn the pump on and hold the trigger at a level lower than the tank, squeeze and hold. What this will accomplish is the sprayer will force the air down and out of the wand. This will cause the pump to change noise once the pump is then primed and will become silent once your sprayer has built pressure back up and is ready to go.

If you are experiencing lost or low pressure on your sprayer, check to see if the filter in the backpack is clogged. Some clogs can be fixed as easily as spraying water into the empty backpack to flush out the filter. Rinse out the tank a few times and see if that fixes the problem.

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Backpack Sprayer Not Spraying (Field King backpack sprayer)

The backpack sprayer may fail to spray because of several issues. To troubleshoot, check the following:

Sprayer wand or nozzle is clogged

To resolve this problem, remove the nozzle from the wand. Flush wand, shut-off and nozzle with clean, cold water.

Dirt or debris in the shut-off valve

To fix this, follow these instructions:

The valve is located inside the handle, so the first step is to depressurize the sprayer and release all liquid from the wand.
Then unscrew and remove the shut-off assembly from the black rubber grip and hose.
Remove the white filter inside the handle and flush with clean, cold water to remove any debris.
Then flush cold water through the shut-off poppet (metal rod located within yellow nut).
Press the shut-off lever (like you would as if you were spraying) for the poppet to open and allow the water to flow through.
Lastly run cold water through the handle and hose.

How Do You Unclog a Backpack Sprayer?

To unclog the shut-off valve, follow these steps:

The valve is located inside the handle, so the first step is to depressurize the sprayer and release all liquid from the wand.
Then unscrew and remove the shut-off assembly from the black rubber grip and hose.
Remove the white filter inside the handle and flush with clean, cold water to remove any debris.
Then flush cold water through the shut-off poppet (metal rod located within yellow nut).
Press the shut-off lever (like you would as if you were spraying) for the poppet to open and allow the water to flow through.
Lastly run cold water through the handle and hose.

To unclog the sprayer wand or nozzle:

Remove the nozzle from the wand
Flush wand, shut-off and nozzle with clean, cold water.

How to Clean a Backpack Sprayer

Milwaukee Battery-Powered Backpack Sprayer

Clean dust and debris from vents. Keep handles clean, dry, and free of oil or grease. Use only mild soap and a damp cloth to clean, since some cleaning agents and solvents are harmful to plastics and other insulated parts. Some of these include gasoline, turpentine, lacquer thinner, paint thinner, chlorinated cleaning solvents, ammonia, and household detergents containing ammonia. Never use flammable or combustible solvents around tools.

Cleaning Electrical Contacts

Keep electrical contact posts between the bottom of the tank and base clean. Make sure to remove the battery pack from the tool before cleaning electrical contacts. Clean contacts with a dry, clean cloth only.

Cleaning the Tank, Hose, and Spray Wand

If there is any liquid left in the tank after spraying, the tank should be drained before cleaning. Do not store chemicals in the tank. You should clean the tank before using another liquid. Do not mix liquids. Follow the chemical manufacturer’s instructions for proper cleaning, storage, and/or disposal of excess liquid.

To Clean:

Make sure the battery door is closed securely.
Remove the tank from the base and set on a level surface away from the base to prevent exposing the base to harsh chemicals.
Remove the tank and fill it about one-third full with clean water. A small amount of mild household detergent may be added. Replace the cap and gently slosh the water around the tank.
Install the tank and spray the water until the tank has been emptied. Make sure to direct the spray toward an area that will not be damaged by residue chemicals in the tank.
Repeat the procedure with clean water until the tank is thoroughly clean.
Wash outside of nozzles and dry with cloth.
Wipe the outside of the tank with a clean, dry cloth.
Allow all pieces to completely dry before reinstalling parts and storing the unit.

Solo Backpack Sprayer

To clean a Solo sprayer, follow these instructions below:

After spraying, clean the tank thoroughly. If some spray liquid is left inside, drain tank completely.
Pumping causes air to be taken in and the remaining liquid to be discharged. Pump until liquid and air are coming out through the nozzle.
Refill tank with a few quarts of clean water and pump the water out as explained above (if necessary, repeat this procedure several times).
If the shut-off valve is removed, the pump can be flushed quickly. Improper spray distribution is the result of a clogged nozzle, which is easily removed and cleaned.
You may use soap and water may to clean tank.
Do not use aggressive cleaning agents or abrasives.
Follow the recommendations of the chemical manufacturer for disposal of waste water and chemicals.
Activated charcoal in liquid or other form may be used to absorb chemicals in tanks or spills.

Note that when cleaning the sprayer after working with hormone weedkillers, you should follow the instructions of the herbicide producers. Neutralize with activated charcoal. (Example: add 0.35 oz./1 g. of activated charcoal to 1.7 pint/1 liter of water and leave this detergent in the tank and the lines approximately 24 hours.) This is very important if other chemicals should be sprayed as the residues of the herbicide may damage susceptible plants.

Cleaning after application of products containing carbolineum, if they are not water soluble, should be done with a 5% soda lye having a temperature of 104°F (40°C). Rinse with plenty of clean water.

How to Prime a Milwaukee Backpack Sprayer

To prime your backpack sprayer follow these steps:

Fill the tank at least three-quarters (¾) full with water
Make sure the pump cylinder is completely submerged
Let the water sit in the tank for a few hours (2-3 hours)
Once the unit has sat for that amount of time, turn the switch to the “ON” position

“l” indicates the on position

“0” indicates the off position

With the switch turned on, place a rubber band around the trigger of the shut off assembly to hold the trigger open (as if spraying)
Let the unit run for 12-15 minutes

Note that the spray may not come out right away as the pump may run but it may not build pressure. This is acceptable and part of the priming steps.

A strong stream should be released at the 12-15 minute mark
The unit is now cleaned and primed and ready for use

How to Adjust a Backpack Sprayer Nozzle for Different Spray Patterns

Sprayer nozzles are responsible for converting the substance(s) in your tank into droplets, regulating flow, and dispersing the spray in a desirable pattern based on the crop you are spraying and what you are spraying the crop with at that time.

There are many types of nozzles and spray patterns available; your best choice will depend on the application. Three of the most common types are:

Flat fan
Cone spray
Streaming nozzle

When using the adjustable nozzle, you can adjust the spray pattern as desired by tightening/loosening the nozzle.

Why is My Battery Powered Backpack Sprayer Not Working?

If your battery-powered backpack spray is not working it may be due to several issues. The following are troubleshooting steps you can try for a Chaplin battery backpack, but can also be effective on other units:

Step 1: Check the Wire Connections

Make sure the tank is empty and not pressurized
Loosen the bolt in the front of the sprayer
Slowly pull down the switch housing
When the wires are visible, make sure they are connected near the pump and at the switch
Once verified the wires are properly connected, reattach the housing to the tank in the same manner the housing was removed

Step 2: Clean Your Filters

There are two (2) filters that need to be cleaned regularly. The first filter is located in the body of the shut off assembly. The shut off assembly is the handle with the red grip and trigger that is squeezed to create the spray. The second filter is in the tank, next to the pump.

Shut Off Assembly Filter

Unthread the shut off assembly from the hose
In the body of the handle is a white filter
Remove the filter and soak in warm soapy water to clean any debris or chemical residue
Once cleaned, this filter reattaches the handle to the hose

In Tank Filter

Looking inside the tank from above, there is a black stick with a finger hole
Pull this stick straight up
Soak the filter in warm soapy water to clean up any debris or chemical residue
Once cleaned, slide the filter back into place alongside of the pump

Step 3: Priming your Pump

Fill the tank at least three-quarters (¾) full with water
Make sure the pump cylinder is completely submerged
Let the water sit in the tank for a few hours (2-3 hours)
Once the unit has sat for that amount of time, turn the switch to the “ON” position

“l” indicates the on position

“0” indicates the off position

With the switch turned on, place a rubber band around the trigger of the shut off assembly to hold the trigger open (as if spraying)
Let the unit run for 12-15 minutes

Note that the spray may not come out right away as the pump may run but it may not build pressure. This is acceptable and part of the priming steps.

A strong stream should be released at the 12-15 minute mark
The unit is now cleaned and primed and ready for use

How to Calibrate a Backpack Sprayer

It is important to calibrate your sprayer to avoid spraying too much or too little, but rather just enough.

There are two ways to calibrate your backpack sprayer – the band method and the spot method. These are used for band and spot spray applications respectively.

To calibrate your sprayer for band application, follow these steps:

Set up a calibration area that has a similar landscape to the area that you are intended to spray. The size of the calibration area is typically recommended to be 340 square feet.
Choose the right sprayer nozzle for the application.
Determine the width of your spray nozzle’s pattern (the band width). If using a sweeping motion to spray from side to side, make sure to measure the full width of the pattern.
Divide 340 by the width of your spray pattern to determine the length of the calibration area. If your spray pattern is 7 feet wide, then the length of the calibration area will be 340 / 7 = 48.57 feet. This means that the size of the calibration area will be 48.57 by 7 feet.
Fill your backpack sprayer with water.
Spray the entire calibration area. You should walk forward at a comfortable speed. Use the backpack like you would in the field. While spraying, pump the pesticide at the recommended pressure.
Measure the time it takes for you to cover the entire testing area. You may do a few passes and take the average time for a more reliable result.
Prepare a measuring container. Spray water into the container for the same time it took you to cover the test area. Pump the sprayer the exact same way as during your test pass. The amount of water collected in the measuring container in ounces will be equal to the gallons per acre required for your land. If you collected, say, 20 ounces, then you will need 20 gallons per acre.
Check the package of your spray to find out how much you need to apply per acre of land. This is to calculate the necessary amount of spray per gallon.

For an easier calculation, determine how many ounces of pesticide you will need per gallon of the mix. Simply divide the required amount of spray per gallon by the total amount of spray.

To calibrate for spot application, follow these steps:

Lay out a 340 square feet calibration area that has vegetation similar to what you will actually be spraying.
Choose the proper nozzle, fill the sprayer with water, and pump it to the right pressure throughout the test. You may practice outside of the calibration area to check whether the sprayer is set up properly.
Mark the water level in the sprayer before starting.
Treat all vegetation in the calibration area.
From a container with a known amount of water, refill the backpack sprayer up until the mark. Check how much water it took to refill the sprayer. This will be your gallons per treated acre.
Calculate how many ounces of pesticide to add similarly to step 9 of the band method.

How to Disassemble a Solo Backpack Sprayer

To disassemble a Solo piston pump sprayer, follow these steps below:

Lock the shutoff valve in the open position prior to starting the disassembly.
To disassemble the pump, first remove the handle by using a 13 mm socket wrench to remove the handle bolts.
Once you have removed the handle lay, the sprayer on its back.
Then loosen the screw securing the hose clamp using a flat head screwdriver or Allen wrench. Remove the sprayer hose.
Now you can loosen the screw on the protective cap that covers the pump using a flathead screw driver and then remove it.
Then you will rotate the pump shaft to make the two levers bolts accessible. Remove them with a 6 mm Allen Wrench.
Next, pull out the piston and examine it. If it is scratched it may need to be replaced. If the piston is good, push off the Viton collar with your thumb and install the new one.
Now remove the piston cylinder by turning it counter-clockwise. Examine the inside of the cylinder, if it is scratched it may need to be replaced. Otherwise, remove the valve plate on top of and the two O-rings on the outside.
Once those are off, install the new valve plate and O-rings. Be sure the O-rings are properly seated in their grooves.
Now remove the valve plate on the inside of the piston cylinder. To do this, use a pair of needle nose pliers to pull out the red pin holding it in. Once out, put in the new valve plate and reseat the red pin.
To remove the pressure cylinder, first remove the screw securing the clamp at the base with a flathead screwdriver and then it can be removed.
Now, unscrew the tank lid and remove the filter basket. If your sprayer has a pressure control knob, remove that as well.
Next, get the pressure cylinder out. Wiggle and push it down through the bottom of the tank. If it won’t come out use a block of wood and a mallet to pound it out. Be sure to place a rag between the block of wood and the pressure cylinder.
With the pressure cylinder out, you will now remove and replace the O-ring at the bottom.

Why is My Solo Backpack Sprayer Not Working?

If have a Solo 414 and is not quite working as it should, there are several things you can check to try resolve the issue:

Check for an airlock in the line and pump.

Air can get locked in between the tank and the trigger, but there is an easy fix for this. Simply turn the pump on and hold the trigger at a level lower than the tank, squeeze and hold.

What this will accomplish is the sprayer will force the air down and out of the wand. This will cause the pump to change noise once the pump is then primed and will become silent once your sprayer has built pressure back up and is ready to go.

Check for filter blockages inside the tank.

If the first step has not worked, the next thing you need to try is check the filter inside the tank for any blockages. Firstly, remove the lid of the sprayer and then the filter basket. Looking in the tank, you will notice a suction line coming from the bottom of the tank.

At the bottom of this suction line is a small, easily-accessible white strainer. Simply remove this strainer, check for blockages and clean it. While you are inside the tank, also make sure to check the top of the suction line and be sure that small clamp located there is tight.

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Make sure all clamps are tightened.

If the above steps have not provided a solution, the next step is to ensure all clamps are tightened within the housing of the sprayer pump. Located on the sides of the sprayer are 4 x screws, holding the pump housing together with the sprayer’s tank. Remove these screws and pull the pump’s housing away from the tank, exposing the battery powered pump.

Upon doing this, you will notice a hose connecting the tank to the pump. Simply make sure that the clamps at either end of this hose are connected and tightened. This will ensure that no air will enter the pump, causing loss of functionality.