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Posted for bob mackey <n103md@yahoo.com>:
Recently, I found a crack in the brake caliper casting as shown in the attached photos. The bleed valve is threaded into an NPT tapered hole. Apparently tightening the fitting generated enough tension in the aluminum casting to
initiate a fracture. I noted spongy brake feel on one side, and leaking fluid. The metal thickness where the fracture occured is 0.057" (1.46mm). That's thinner than the walls of any of the NPT fittings I have laying around, and
in a castable aluminum alloy. Perhaps I overtightened the fitting at some time, or maybe that's just not enough metal to support this type of fitting when properly torqued.
The failed caliper was made around 1986-88.
A later Matco caliper that I pulled off our partially-built 320 did not have a machined face. Instead it had the Matco name in the casting, and somewhat thicker metal around the two NPT fittings.
One more thing to look for when you inspect your brake system.
I will contact Matco tomorrow and see if they are interested in doing a post-mortem analysis.
-bob mackey
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