As many of you already know I recently completed a rear disc conversion on my '84 Z28. Although the brakes work great, I am still a little bit confused about a couple of things:

*I have always believed that the proportioning valves were different, internally, between the two types of rear brakes. (disc/drum or disc/disc) Recently an old and wisened friend [a well respected dirt track racer for the last thirty years] of mine explained to me that on our cars (non ABS) the valve is actually only a junction block and does not deliver differing amounts of pressure front-rear.

*Secondly, this same person explained to me that the rear disc brake master cylinder was different in that there is no check valve in the line from the M/C to the rear brakes as opposed to the rear drum brakes that utilized this check valve to maintain a small amout of pressure in the rear line to take up the slack inherent in a drum brake system. (Since disc pads remain so close to the disc there is literally no take-up in the pedal travel)
Furthermore he told me that you could remove the check valve from the drum M/C and it would work fine for the disc.

Can anybody elaborate or colaborate any of this information/misinformation? I do not wish to mislead anybody but I am just curious if there is someone out there that can back this up.

BTW, my M/C and prop valve came from an '89 Firebird. Unfortunately, I was unable to determine what brakes this car had due to missing RPO sheet and no rear suspension or E-brake brackets which were slightly different on the rear disc cars.



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'84Z28/'92 305TPI
'92 complete rear suspension/PBR disc/Posi/3:42
Light weight 34.5mm stabilizer bar in front
23mm sta. bar in rear
rear lowered 1" by cutting 1 1/4 coils of '84Z28 springs
comp cam, flowmaster
TB coolantbypass/shutoff valve to heater core
MAT relocated
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!

freshly painted OCT. 2000 Teal with two white stripes on the hood and deck lid

LOW performance people should drive low performance vehicles!

 

The following links may or may not work on a 3rd Gen F-Body. If you do this mod on your car, do so at you own risk.
http://www.b-body.net/Technical/Susp...Mod/index.html
http://www.b-body.net/Technical/Susp...eMod/bolt.html
http://www.b-body.net/Technical/Susp.../feedback.html

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Rick89IROC
Black 89 IROC-Z Convertible 305 TPI
14.07 @ 97.9
DGGM 95 Impala SS
13.78 @ 98.28

 

As I've read it and understood it:

The disc/drum prop valve have a metering valve on the back, facing the firewall, and covered completely by a rubber plug.

The disc/disc prop valve doesn't have one. It mainly provides the "split" for two front/one rear brake line, and works the pressure warning light.

My dad has old 1976 Ford service manuals, and busted them out for me when I did my axle swap. It showed the internals of three prop valves- two of which looked like my GM ones! I can't remember the name associated with those. Maybe I'll hunt that page up and scan it in. Is that possible? Ford would use the same prop valves as GM?

For 89-92 cars, both used the "original" disc/drum (J50) master cylinder, so I won't go into describing those. (Oh, wait, maybe that helps a bit for your question?)

For 82-88, there were two m/c's. The main only difference between the two that I knew about is that the disc/drum m/c has a 24mm piston bore. The disc/disc m/c has a 25.5 (1 inch) piston bore.

I don't know about that check valve issue. I seem to remember the GM rebuild procedure for the master cylinder being the same between the J50 and J65 (disc/disc) brake systems, and I think both mention a non-servicable valve.


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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!

 

I installed a 92Z28 rear in my 89 RS convt that was equipped with drum rear brakes. The brakes felt ok. I then swapped in the 92 master cylinder and prop valve and I felt a big difference. I think there is a difference, though I am still not happy with my rear discs and probably will install an adjustable prop valve.

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Rob P
89RSconvtZZ4TPI
92Z28convt5spd
71Impala convt 402BB
BETTER DRIVING THRU SUPERIOR HORSEPOWER!