Another weakness is the secondary, "dust boot" which is only marginally effective. Abrasive material can work around the inner edge of the boot and lodge in the space between the caliper bore wall, the brake piston and both seals. As the brake pad wears, the piston slowly moves outward. The primary seal moves along the caliper bore trapping brake dust, or whatever, under the seal. Over time, this can abrade the seal enough that it begins to leak.

If the car sees use where salt is used to melt road ice, salt finds its way around the secondary seal. The caliper bore beyond the primary seal corrodes in short time due to the reactiveness of the salt. As the brake pads wear and the seals move out, they encounter the salt-induced pits and begin to leak.

It took 14 years, but Chevrolet indirectly admitted the secondary seal problem. Starting in 1979, they recommend adding beads of RTV silicone sealer in the secondary seal groove on each brake piston and, after the piston assembly has been installed, over the gap between the dust boot’s outer ring and the caliper half.

(...)

Earlier we discussed at length the substandard sealing system used on the ’65-’82 brake pistons. In the last few years of that platform’s production, Chevrolet became aware of this problem and in factory Service Manuals for the ‘79-’82 model years, recommend a modification using silicone RTV sealer.

We disassembled each caliper, carefully pried out its dust boots, removed the piston assemblies and performed this field fix. A bead of Valco Cincinnati Silicone Sealer was applied to the groove into which the outer piston seal or "dust boot" seats and, once the piston assembly has been installed into the caliper half, you apply a bead of RTV to the parting line between the cast iron of the caliper and the dust boot. Valco Silicone comes in several colors and we used silver because it matched the silver coating on the Vette Brake and Products calipers.

Then, we reassembled the calipers. Use a torque wrench when tightening brake caliper cross bolts. Bolts on front calipers must be tightened to 130 ft-lbs. and bolts on rears to 60 ft-lbs. Many books (including some factory manuals) have incomplete torque specifications for the cross bolts–sometimes only the front number is listed. Inadvertently tightening the rear cross bolts to the front figure will damage the calipers.

Even if you do not disassemble calipers to RTV the pistons; we suggest checking cross bolt torque. Many rebuilders use non-torque sensing, impact wrenches to tighten the bolts. They will tell you they adjust the impact such that it tightens the bolt to a specific torque, but that method of controlling bolt tightness is inaccurate. Over the years, we have checked many rebuilt calipers and found bolts as much as 50% under the recommended specification.

Incorrectly tightened cross bolts pose a substantial safety problem. The caliper flexing that results from normal brake use will cause undertightened bolts to loosen. Eventually, the caliper halves separate and the o-ring sealing the brake fluid passage running across the caliper blows out causing a massive fluid leak. This is most likely to happen under maximum braking, so a failure of this nature would be catastrophic causing loss of control of the vehicle. Regardless of the source of the brake calipers; check the torque of all cross bolts.