When brazing dissimilar metals and the filler metal cracks after it solidifies, the different coefficient of expansion may put the filler metal in tension just below the liquids temperature during cooling. This sometimes occurs in a copper-into-steel joint.
The copper expands and contracts at a greater rate than the steel. Brazing alloys are stronger in compression, so a steel-into-copper assembly would help alleviate the problem.
Brazing steel or other ferrous metal with an alloy containing phosphorous can lead to formation of a brittle phosphate prone to cracking. Braze ferrous metals with non-phosphorous bearing alloys.
Excessive joint clearance can lead to filler metals cracking under stress or vibration. Make sure clearances are held to .002 inches to .006 inches at brazing temperature, depending on the alloy.
Source: The Serviceman's Guide to Soldering and Brazing, J.W. Harris Co.
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