First, I highly recommend having a spare spray pump on hand for self-propelled and pull-behind sprayers. Yes, it's expensive to have a spare pump, but the window of opportunity for spraying each spring is SO small that it's much more time-effective to bolt on a new or rebuilt spray pump than it is to spend time installing a seal kit in the leaking or faulty pump.
Second, mid-winter is a good time to rebuild any spray pumps that got stripped off a sprayer last spring and tossed under the workbench to be rebuilt, "when I get time." It's not difficult to rebuild spray pumps---it's just a matter of getting the right parts, having the right tools, and having enough patience to do the job correctly.
The exact procedure to rebuild spray pumps varies slightly from model to model, but in general:
-Before doing ANYTHING, use the model number and other relevant information on the pump's information tag to get the correct seal kit/rebuild kit from your local spray pump supplier.
-Wear rubber gloves of some kind while working on the pump. Even if you triple-flushed it with water, there will be residual pockets of chemicals you'll disturb during disassmbly and re-assembly.
-Before disassembling the pump, use a center punch or cold chisel and hammer to make reference marks on the halves of the pump housing and whatever hydraulic motor or belt drive-assembly that powers the spray pump. The goal is to mark the various components so you can re-assemble them in the correct orientation so the hoses and fittings will line up correctly when you put the pump back on the sprayer. (Center-punched or chiseled marks won't wash or rub off like alignment marks made with paint sticks or Magic Markers. Trust me on this.)
-The instructions that come with seal kits/overhaul kits are usually pretty detailed. Read them carefully, follow them completely. Shortcuts generally lead to leaks.
-When in doubt, replace parts. If the impeller looks funky, replace it. If the shaft on the drive motor/belt pulley shows corrosion, replace it. If the inside of the pump housing is corroded and severely pitted, replace it. That is, unless you like doing things once in the winter and again in the spring.
-Many pumps have a ceramic seal pressed into the pump housing through which the impeller driveshaft runs. Instructions with rebuild kits say to break that ceramic seal with a hammer and punch to remove it. Don't break it---CRUSH it, SHATTER it, DEMOLISH it into many tiny pieces. If you are timid and break it into only two or three pieces and then try to pry those pieces out of the cavity, those brittle, sharp-edged ceramic shards will wedge against the driveshaft or pump housing and carve scratches in those surfaces that need to be absolutely smooth. Once you've crushed the ceramic seal into small enough pieces so they literally fall out of the housing when you invert it, flush all the dust and broken pieces out of the housing with brake cleaner before reassembling the pump with a new ceramic seal.
If you can do basic maintenance on a tractor or farm equipment, you can rebuild a spray pump. Move slowly, follow the instructions, and you'll have a back-up pump ready to slap into place and keep you spraying next spring.