Just moved into a 30 year old condo. The diverter in my tub/shower is faulty and diverts only about half the water to the shower head. The plumber found that the core of the diverter is frozen, and was unable to break it free by force. He offered to return with a product called Crawl that might solve the problem. Does anyone here know this stuff, and can recommend it? Or any other product for this purpose? Thanks.