Are they all the same? Is there a differance between a rear disc and drum rear valve?
Could you use a disc valve if you have drums?
I might be getting a car that originally had rear disc that someone put drums on--I have all the parts now but I just wanted to know what to keep before I sell it.

 

There is a slight difference between the two. I believe it is a simple matter of the valve limiting the amount of fluid going to the rear, especially in rear disc applications. Too much to the rear would cause a potentially dangerous lock-up of the rear wheels in certain low traction conditions.

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'84Z28/'92 305TPI
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23mm sta. bar in rear
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comp cam, flowmaster
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Ram-Air(home-made) gutted air-box
IROC ground effects/wheels GY245/50's
700R4 superior shift kit/ corvette servo(excellent shift kit BTW) Rebuilt engine and transplanted it myself. Love this engine!
LOW performance people should drive low performance vehicles!

 

So then you really couldn't use the same valve on either setup?
Thanks for the response!

 

There was a difference in the drum brake and disc brake p.v.'s on pre-89 F-body cars. It is easy to identify the difference visually. The drum brake p.v.'s have one brake line coming out of the front of the p.v. and another from the bottom. The disc brake ones have two brake lines coming from the bottom of the p.v. The newer 89-92 F-body cars have the same p.v. for both drum and disc brake applications. I have heard a rumor that the brake master cylinders are different for drum and disc applications as well, but haven't confirmed it yet. Good luck, Lon.

 

Reason I'm asking is I'm trying to buy an 88 Formula that originally had discs but was switched over to drums--I have a pv from my 89 that had discs--don't know if I should sell it or use it on the 88. Don't know which pv the 88 is using. I plan on switching the car back to rear discs.
Opinions anybody? Keep it(pv) or sell it?

 

Lon, I've confirmed the master cylinder issue a few times, here's the results. You can also verify this data through places like carparts.com and wrenchhead.com.

82-88 = 2 different master cylinders for drum/disc (J50) and 4-wheel-disc (J65)

89-up = Same master cylinder for original 82-88 J50 application used.

As to visual differences of pre-89 prop valves, my old '86 drum brake PV had two brake lines coming out of the bottom. The '84 J65 4-wheel-disc PV had one brake line out the bottom, and one out the front... opposite of what you mentioned.

I think the difference for prop-valve-appearance has to do with the year. Seems the J65 valves with a forward-facing "rear brake" port are on pre-85 cars. All the 82-84 F-bodies I saw (J50, J65, missing the rear axle, etc) had a forward-facing rear brake port. I'm not 100% sure though, I haven't seen a rear disc setup from 85-up. Talk about confusing, eh? (laughs)

Crap- I just realized my '86 J50 prop-valve-to-rear-brakes-line (which entered the prop valve from the bottom) has been outside, for the past three months. I can't remember if I capped the ends- damn!!

As to other prop valve differences, the J50 units had an externally-accessible valve under a rubber cap, meant to equalize pressure. Read about it here:
https://www.thirdgen.org/messgboard/...ML/000438.html


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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 22, 2000).]

 

Tom P, my 88 Iroc is slightly torn apart at the moment. I swapped the 3.45 9-bolt from it to my 90 RS. I began having the p.v. imbalance problem then. I swapped in the p.v. from the Iroc, even though both were the dual bottom feeders. Still same problem. Today I looked at the master cylinder from the Iroc. According to your info, being a pre 89 with disc/disc it SHOULD have a J65. Nope, it has J50 casting number. I don't know if it has been swapped previously or not. Unfortunately I have lent my 88 Helms and my 90 Helms doesn't cover the old style calipers, etc.

 

Hm, interesting. Maybe the 9-bolts got different M/C's? Any parts counter guy will show the J50/J65 split between 82-88 on his computer.


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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
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Tom P: More info I have confirmed regarding the brake master cylinders. Yes you are correct that the A65 was used on the disc/disc setup prior to 89. I went to Pep Boys to get the correct A65 m.c. for a 88 Iroc with disc/disc. I returned with it in hand and pulled off my m.c. only to discover it was exactly the same as what I had just picked up. It had the same p/n 18006814 cast on the body, but lacked the A50 casting number. Then we called the local dealer and learned that GM has superceded the A65 with the A50 for that application. We were told the A65 is now discontinued. It looks as if I will need to go with an adjustable p.v. to solve my problem.

 

Discontinued? Ouch! I got my rebuilt one from A1-Cardone through a local parts store; I think they sell new ones, too.

Are you saying the 50/65 was cast into the m/c body? Shoot, I never looked at my stuff. I'll have to check those out later!


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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
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Tony, The "A50" is cast into the 3 master cylinders I have (my original 90 RS, 88 IROC and a spare from another 90 RS I just picked up). The rebuilt one I got from Pep Boys that was supposed to be for a 88 Iroc with high performance disc/disc didn't have "A50" cast in it, yet had the same p/n as I described. I have yet to see a A65 m.c.

 

I'll have to check my master cylinder before I leave work to see what's cast into it.. I'll put the info up here when I get it.

[edit] Ill have to check my original disc/drum master cylinder (in the basement) too...

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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
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[This message has been edited by TomP (edited December 05, 2000).]