Briggs and Stratton 11/5 hp Snow Blower?

I have a Sears Craftsman snow blower with an 11/5hp Briggs and Stratton motor that I purchased in Oct.06.( MODEL NUMBER 52431 ) The blower is kept outside covered with a snow blower cover bought at the same place. In Nov.07 I went to use the machine and I found the block was full of water and froze. The block was replaced under warranty. The mechanic said that it was a common problem with that model, the water gets into the motor through the muffler. I told him it was covered up for the summer and asked, how would the water get in, he said it was probably sweating under the cover. Over the summer of 2008 I checked and ran the machine weekly, it was all good. The snow came it worked well, until Jan,7,09 the motor would not turn over. I took the blower into Sears to get it looked at, water got in again, broke the Push Rod!!!! Again they tell me the water gets in through the muffler, but, Sears tells me its not covered under warranty. This is a reoccurring problem. Can anyone relate to this and is there a solution ???