skinny z

I've had a couple of questions answered regarding the different codes for brake options.
I need a little more detail if anyone has experience.
My original master cylinder, a 1986 disc/drum Sport Coupe has a small piston bore.
My potential replacement was scooped from a 4-wheel disc Camaro with a manufacturing date of 07/85. This master has a larger piston bore diameter.
It also has inverted flare fittings. There are adaptors to connect to the bubble flare lines that were on the car.
I suspect this rear disc axle was swapped into the donor car at some point (indications are that the donor was a hot rod as it still had headers). The master is probably a swapped in part as well. Interestingly, the prop valve is the cast type.

Does the larger piston bore make this the master cylinder for the J65 option code?

Does the inverted flare (and I suspect SAE rather than metric threads), make this an early 3rd gen (82-84/5) part?

I understand the cast prop valve is an early production piece as well.
I'm still looking for a J65 prop valve (rather than an aftermarket adjustable unit).

Apeiron

I don't remember which size was which, but the only difference between the J50 and J65 is the bore size. I think the small bore was the J65. They're interchangeable in either application, and GM released a bulletin advising dealers to do this to address customer pedal-feel concerns.

If it has SAE fittings, it wasn't original to the car it came out of. If it was from a thirdgen, it would have to be an 82-83.

skinny z

I have several master cylinders in the shop and all but two have the smaller piston bore. Even though the two are interchangable, which do you think would have been likely to be factory issue on disc/drum cars? From my scrap yard experiences, the masters I've salvaged from disc/disc cars have all had the larger bore. I'm inclined to think the J50 is the more common of the two seeing as I have a half dozen of them and they have the smaller bore. This I'm not sure of though.
As for the SAE fittings on the master I just picked up, I'm pretty sure the disc rear end was swapped in and the master salvaged from an early model like you suggest. It has the larger diameter piston.

skinny z

In addition to determing (precisely) which is the J65 master cylinder, I'm also looking for the bulletin that GM passed to around to their dealers regarding the "pedal feel" that Aperion mentioned. I've read it before and that I had it stashed in my car stuff documents however I can't find it there or anywhere at TGO.

Apeiron

Last post here https://www.thirdgen.org/forums/brak...ing-1le-3.html

JamesC

iTrader: (5)

It's also in the 10-Bolt FAQ.

JamesC

skinny z

Thank you.

skinny z

The question still remains.
Which master cylinder has the larger piston bore? The J50 or the J65?

skinny z

Here's an ancient post I lifted. Still no reference in how to identify what one has in inventory.

88 350 tpi formula
Senior Member


Join Date: Dec 1999
Location: WI,USA
Posts: 3,022
Car: 89 FORMULA 350
Engine: 8 cylinder
Transmission: one with gears
Axle/Gears: BW 3.70


Classifieds Rating: (5) I got the cross ref. for my 88 and it shows as follows...


Master Cylinder

Part# Discription

18009364/ 4-WHL DISC(J65),(ECT 1LE)88-88

18009374/ Vac PWR BRK(J50) 82-88

18009374/ 4-WHL Disc(J65),(1LE) 88-88

HYDRAULIC BRAKE COMBO/MONITERING VALVE

Part# Discription

14089495 VAC PWR BRK (J50) 86-88

14089497 4-WHL DISC BRK (J65) 86-88

If you notice above the master Cylinder is the same for 82-88 drum and 88-88 1le and some of the all wheel disc systems.

skinny z

Master cylinder aside, is there a way to differentiate between a J50 and J65 proportioning valve? I have several on my bench and all are identical on the outside (except for the early cast model).