If you have metal eyelets on your head, check them to make sure that they aren't scored. When mine get that way they cause the string to heat up and break. I simply rotate the metal eyelets a little so the string does ride on the scored area.
Try TERA Pump. Makes all types of transfer pumps and not china junk. Decent quality and made in S. Korea (not US but at least our ally). I shop it on a site called PublicSquare.com. It's a good site and tries to maintain conservative values.