Open your backpack up and see what amp the pump is and put in the same amperage as what written on the pump label. It`s could be a 3 or 5 amp fuse. I have used many of Hewit pumps and his stuff is cheap but still good. I have his 24-volt booster pump for my pure operating every day for 8 years never missing a day yet. To many only think things are good when it cost a shed load of money Sure Flow must be the most used pump and yet they give more trouble than most.
P/s most backpacks are not made well I have the Bright yellow from Window Cleaning Warehouse and I have rewired it and it`s only worked well since it`s rewired. Chucked out all the switches and battery and wiring and replaced the lot with soldered wiring. switches and a 12 amp hour battery.