Okay, this has nagged me for years. I read a children's book circa 1970, set in the western US in early-mid-20th century. Horses and trucks both. A mystery. Protagonists two boys and one girl. There is a lost treasure (a mine?) in a hidden valley somewhere in a small mountain range. One character takes a job as a helper to a peddler (a handyman?) who drives a van throughout the region. Eventually the main characters discover that the hidden valley can only be found by swimming underwater (through a pool or stream?).
Probably not a classic, but I'd love to re-read, enjoy and exorcise!