Brake Tech, what do all the numbers mean..

[Captkaos]

There are 2 style calipers and rotors on these trucks after 1980,  Standard 1" and HD 1.5"  They don't interchange.

For 1/2 Tons..
JB1 - manual brakes, 1" rotors
JB3 = power brakes, 1" rotors
JB5 = power brakes, 1.25" rotors

JB1 BRAKE, MANUAL, DISC/DRUM, 4300 LBS
JB3 BRAKE, POWER, DISC/DRUM, 4600 LBS
JB5 BRAKE, POWER, DISC/DRUM, 6400 LBS
JB6 BRAKE, POWER, DISC, DRUM 7200 LBS
JB7 BRAKE, POWER, DISC, DRUM 8400 L85
JB8 8RAKE, POWER, DISC, DRUM 10000 LBS
The only rear ends that have 13" brakes are the JB7 and JB8 brakes on a 14 bolt full float.

JD5: Brake, Dual Power, Disc/Drum, 6,400 Lbs GVW (Dual Diaphragm Vacuum Booster)
JD6: Brake Hyd Power, Disc/Drum, 7,200 Lbs GVW (Hydro-Boost Booster)
JD7: Brake, Hyd Power, Disc/Drum, 8,400 Lbs GVW (Hydro-Boost Booster)

10 or 12 bolt [rear axles] had either 11.15" x 2.75" for the JB5 or 11.15" x 2" for the JB3.



1973-1980
If your master cylinder looks like this then you have the JB1 (Manual brakes)


If your master cylinder looks like this

then you have
JB3, JB5, JD3 or JD5

If your reservoir looks like this

then you have a JB6

If your reservoir looks like this

Then you have a JB7





1981-1987
If you have a reservoir that looks like this

Then you can have a JB1, JB3, JB5 or JB6
JB1  plastic reservoir will have a bore diameter of 1.417"
JB3 and 5 plastic will have a reservoir bore of 1.574"
If your reservoir looks like this

Then you have a JB7 or JD7
If your reservoir looks like this

Then you have a JD3 or JD5

If your reservoir looks like this

Then you have a JB8

More Brake system info
http://forum.73-87chevytrucks.com/smforum/index.php?topic=33548.0