JL8 master cylinder

[KurtS]

Looking for confirmation of the JL8 master cylinder code. Is it US or something else?
JL8's don't have the residual pressure valve for the rear brakes. Maybe that was part of the master cyl assembly and the master cyl's are the same.

[bcmiller]

 I believe the JL8 master cylinder used a larger piston diameter. Possibly the same as the 4 wheel disc Corvettes.

[Stingr69]

I can't see how the MC/Booster assembly could be a "US".  The AIM suggests the assembly is different from "US" as it specifically refers to a separate parts list to find that item when the option was ordered.  From a functional standpoint it makes sense that it would need to have a different volume for the calipers VS the wheel cylinders.  I don't know what it should be, but I doubt the "US" was used.

[Steve Shauger]

Both JL8's I owned had the standard US master cylinders and worked perfect.

[RS_COPO_Canuck]

I seem to remember they used the US master but deleted the proportioning valve?

[Bryan302]

Several things lead to suggest that the cylinders are different!

#1, on page 126 of John Hooper's book shows a JL8 master cylinder, and is quite obvious that it is not a 309 US cylinder.

#2, the chassis broadcast codes are clearly different, the CU code for a US cyl. and a CX code for a JL8 cyl.  If they were the same, there would not be any need for a different code.

Bryan

[69Z28-RS]

Bryan,

As Kurt suggested earlier, perhaps those 'CU or CX' codes included the complete brake MC arrangement, including (or not) the residual pressure value (in the MC outlet for the rear), the metering valve, and perhaps even the proportioning valve.   If that code includes these additional components (or not), then the 'US' coded (309) MC could have been used for either disk/drum, or disk/disk systems.   I know of an original '69 SS Camaro with JL8 brakes which I need to try to get access to it and take some photos.  If I can do that, I'll post here.

Given the high incidence of the JL8 rear disk brakes having been *moved* from car to car, and *added* to cars not having come from the factory with it, I'm a bit skeptical of most/any? photos which have appeared over the years in books and magazines, as the owners generally 'attest' to the JL8 system being *original* to their cars (whether they know that fact to be true or not!)...

[bcmiller]

Its tough to get rock solid data points now 47 years later on things that were quite rare to start with, but we have to try.

[bcmiller]

At least on Corvettes, I believe power brakes cars got the larger bore (1.125 inch) master cylinders, while non power brake cars got the 1 inch diameter.

Steve Wright posted something in a 2008 thread and it had US for the larger bore power disc use in 69. Not sure if known information has changed now.

[1968RSZ28]

JM's '69 Camaro Z28 Fact Book says the JL8 master cylinder is identical to the standard '69 Z28 master cylinder and is stamped "US".

Paul

[WorkinProgress]

Kurt, all the original JL8 cars I have seen have the US stamped master cylinder.

                                                            - Warren

                                                                 

[68camaroz28]

The only car I ever checked had a BA but that might have been changed when the car was restored in the 90's or even before for some reason. The BA master cylinder with that JL8 car worked excellent. Not sure what differences are between a BA and US but assume minimal.

[69Z28-RS]

From my copy of the Oct '70 Parts and Accessories Catalog, under Sec 4.65   CYLINDER ASSY. - Hydraulic Brake Main...
...
69   Chevy, Camaro w/Disc Brakes   5463751    1  28.65    (1-1/8" dia. cyl.) (stamped U.S.)
69   Chevelle, w/Disc Brakes            5469399    1  35.55    (1-1/8" dia. cyl.) (stamped B.A.)

this raises some questions: 
1) Why does it list both 'Chevy', and 'Camaro' with Disc brakes??  for the US MC?
2) Why does the BA MC (for the Chevelle) show a 'price' which is 20% higher than the US MC?

There *must* be some substantial difference for the difference in price, AND the fact that two different MC are listed even though they each have the 1-1/8" piston bore??  Both are still in the catalog almost a year after the end of production for the '69 models.

[bertfam]

From the July 1st, 1969 "Heavy Duty Operation Parts for Passenger, Corvette and Camaro" manual:



From the June 24th, 1971 "Standard Parts Service and Maintenance Products" manual:



The bottom picture isn't very good, but you can clearly see the 5463751 Master Cylinder code "US"

Ed

[luv2sixty9]

309 MC with US Stamp is the correct one

Dave