What do you mean 'stiff'. You might want to go to an RV store and check out a new one for comparison. If it pumps well with no leaks
, you may want to disassemble it to find if anything is binding. At that time you will be able to determine whether it is worth repairing.
Based on the kit components, it appears to be a simple double acting diaphragm pump. Not much to go wrong, unless it's been abused.