Found out melling had redesigned m55 per gm to save weight. Don't know why. Melling's m55hv has not been redesigned and it works great. Dirt track customers have had the same breakage problem.
M55HV is no longer an option. The 10555 is a better casting, but uses powdered metal gears. Use at your own risk. :roll:
If the pump is more than roughly 2 years or older, you are OK. All the newer stuff is PM. I e-mailed Melling last year and they confirmed this. They also supply Moroso, Milodon, and a few others with gears.