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If this problem only occurs when it is raining, it is due to the belt slipping. When you are going very slow and using the brakes, it puts additional load on the steering pump and the water makes the belt a little bit more slippery.
Check the condition of the accessory belt and make sure that it's tensioning system is holding proper tension. Be sure the tires are properly inflated (this can make a big difference if they are low).
It is not likely that there is a power steering pump problem or speed sensitive steering issue. The weather is just aggravating a problem that is already prone to exist.
I hope I have been of assistance. If so, Please accept.