Hello,
This will probably be a tough one.
HERE is a full write-up I did for my blog. It's driving me NUTS (and it probably didn't do me any good as a kid either).
Love the site. You are doing God's work. Which god though I am afraid to speculate upon.
Take care,
Dan Kelly
This is just a guess but maybe 1972 Disney's Snowball Express.
Wiki article
http://en.wikipedia.org/wiki/Snowball_Express
Pictures
http://bookstevebijou.blogspot.com/2013/12/snowball-express.html
It does line up somewhat with some of your information. Probably not it, but worth a shot.
Good luck.
Just did a keyword search on IMDB. Maybe some other person might recognize your film from the list. We can safely rule out Cujo. lol
http://www.imdb.com/search/keyword?keywords=saint-bernard-dog
I have no idea if this is what you're looking for, I just know this movie exists: http://www.imdb.com/title/tt0066660/?ref_=ttpl_pl_tt