One cause of threads breaking in antique machines can be that the bobbin thread didn't get wound tightly and with even tension. Modern machines have a tension disc that the thread goes through for bobbin winding, which older machines often don't have. To get the bobbin to wind tightly, when you are winding a bobbin pinch the thread between your thumb and forefinger before it goes into the thread guide just before before the bobbin winder. I've found this trick helps me wind bobbins more evenly on modern machines, too, and as an added benefit you can fit more thread on the bobbin.