Differences in A Body brake systems
Interested in knowing the difference between the front disk set up on a US 72 A Body, and a 74 US A body?
Are the rotors a different diameter or thickness?
Yes. The 72 uses a slightly greater diameter, thinner rotor.
Is the Bolt pattern different?
Yes. In the US the 65-72 disk brake used a 4-inch bolt pattern. The 73 and later used a 4½-inch bolt pattern. Please note that the 73 and later drum brake cars still used a 4-inch bolt pattern.
Differences in caliper etc?
The 72 and earlier US disk brake cars used a fixed caliper unit manufactured by Kelsey-Hayes. This design did not require the caliper be removed for renewal of the pads. However, it is less tolerant of rotor runout. The caliper can be easily identified by an external (and vulnerable) balance tube that supplies brake fluid pressure to the outer pistons on the caliper.
By contrast, the 73 and later disk brake cars used a floating caliper design. I'm thinking this was a Delco-Moraine product, but not certain. This design is simpler to maintain, less prone to leakage, and more tolerant to rotor runout.
There were three different proportioning valve schemes used in this era. The first was an external valve near the master cylinder. The second was a proportioning valve incorporated in the Tee on the rear axle. The third, and most likely for the 74, was a combination distribution/metering/proportioning valve located on the firewall. This valve is identified by five hydraulic connections (2 from the master cylinder, 1 from each front wheel, and one to the rear brakes).
