My car originally came with J50 rear drum brakes, but was upgraded to discs in 1985, at which time the master cylinder and proportioning valve were replaced. Is there a way to identify which master cylinder and prop valve are on the car now?

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler

 

The prop valve's kind of easy. The J50 disc/drum units had a rubber-capped valve on the extreme back of the prop valve. This was meant to be pushed in, to equalize the pressure, or something to that effect, when doing certain kinds of brake work.

The J65 prop valves didn't have one. If you have a J65 valve, the only things sticking out should be:

two inlets from master cylinder
electric connector for "BRAKE" warning light
two outlets for front brakes
one big outlet for rear brakes
closed-over port for actual prop valve

If you have a sixth thing, you've got the J50.

As to master cylinder, well, they both use the same reservoir. Unless someone knows what the casting #s mean, you'd have to separate the m/c from the power brake booster. That's no big deal; you don't have to open any brake lines... just remove the two bolts that hold the m/c and prop-valve-bracket to the booster. Then try to pull the whole m/c+prop-valve assembly forward, and measure the bore of the piston. J50 m/c's had a 24mm bore, J65 m/c's had a 25.4mm (1 inch) bore.

You can also tell by looking at the bore.. altho it would help to have one unit as a comparison. Know how the piston's in the center, with the metal housing "lip" around it? Well there was a thicker lip on the J50 m/c's. The J65 lip looks very thin.

[edit] The boosters were the same, so you can't tell by those. If you don't have calipers to measure the bore, you could measure the lip as close as possible, and let me know... I have my '86 J50 m/c (same from 84-88) still in my basement, and could check your #'s.

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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!


[This message has been edited by TomP (edited November 20, 2000).]

 

Thanks, I'll have to have a look the next time I visit the car in storage.

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1984 Silver Z28, 383 cid 4 bolt, ARP Fasteners, 2 1/2" Cowl Induction Hood, 600 cfm Performer Carburetor, Torker II Intake, Performer RPM Heads, XE268H Cam, Magnum Roller Tip Rockers, MSD6AL Ignition, Blaster2 Coil, Recurved HEI Distributor, 700R4, TransGo SK, 2.73:1 Limited Slip Dana 44, Terminator Headers, Custom 3" TIG Welded Stainless Exhaust, Flo-Pro Twister Muffler, 4 Wheel Disc Brakes, Oil Cooler