I have an 85 Iroc Z-28. I am wanting to know what has to be done to convert from rear drum to disc brakes ?

 

Easiest way is to swap rear ends. Also swap the master cylinder, proportioning valve, and grab the brake lines and parking brake cables just in case there's differences (sometimes, but not always).

I'll be doing it eventually, but it's tricky trying to find the combination that I want: 89+ discs, #>3:1 gear ratio, and posi.

 

I swapped discs into my 85 IROC (came with them but were switched out for some reason). The rear end (might as well get a limited slip if you can), master cylinder, pro value, and e-brake cables were all that I needed. The swap might be a good time to stick in some braided steel brake ilnes.

JamesC

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Why does everyone say to change the master cylinder and proportioning valve when you do this swap? I know there is a difference in the size of the bowls between drum and disk master cylinders (drum is bigger), but wouldn't you be able to compensate for that with a new adjustible proportioning valve?

While on that topic, I have yet to install my proportioning valve or master cylinder. I have been driving and stopping just fine for almost 6 months now after I swapped rears (9-bolt posi with disk). Granted, I do not feel that my front brakes are doing all they could be doing... but in panic stops my rears don't lock up and I fear for my life! I will also say that I will install the adjustible proportioning valve when I get around to it... hopefully sooner rather then later.

Get the e-brake cables that came with the rear if you can and do an overhaul (change the fluid, braided steel lines, etc.) before you install it. NOTHING stinks ore then diffy fluid!

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

Yes, you could buy an adjustable valve instead of grabbing one off the donor car. The hardest part would be dialing it in.

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1989 IROC-Z 5.7L

 

I have also done the swap. In my case it was a pre-89 style 9-bolt posi disc rear into my 90 RS. Swapping the master cylinder and proportioning valve depends on what year your car is. Pre 89 disc/drum used a A-50 master cylinder. Pre 89 disc/disc cars had a A-65 master cylnder. From 89 on GM used the A-50 master cylinder for both disc/drum and disc/disc equipped cars. Also, according to my local GM dealer the A-65 has been superceded by the A-50. OK, now on to the proportioning valve question: Pre 89 disc/drum cars had a different (cast cylindrical shaped) p.v. These are easily identified by a brake line coming from the front of the p.v. The pre 89 disc/disc cars (and all cars from 89 on) have a p.v. that appears to be machined from an extruded aluminum bar. It has both brake lines coming from the bottom. Bottom line: For pre 89 cars you will need to change the p.v. when swapping in a new disc brake rear end. Otherwise you may experience brake bias problems. The brake light will come on and there will only be roughly half the normal braking available. You can either CAREFULLY re-bend the brake line that previously went to the front of the p.v., or replace the line. Some people still have a bias problem and have used an adjustable p.v. to solve it. Also the pre 89 cars have a different emergency brake handle assembly which has an adjustable center rod at the saddle where the 2 cables come together. The 89 and later cars have no adjustment at the emergency brake assembly, the disc brakes are adjusted at the calipers themselves. Also the drum brake and disc brake emergency cables are different as well. Sorry this was so long, but I hope it helped with your brake swap questions. Good luck, Lon.

 

The reason for the master res and valve is because it takes alot more power to use disk that drum. Drum brakes are actually more effecient, but they lack cooling and suck when they get wet. I have a 65 pontiac with aluminum brake drums, they help alot.

 

Whoa whoa whoa, Lonsal your telling me that If I have a pre 89, Disc/drum Car( 85 SC rear drum), I can use My existing MC with a Post 89( or pre 89 ? ) Rear disc Setup, and only have to replace The Pv ? ( and possibly some line issues ? )


This is news to me, Good news, but News all the same, please elaborate.


And when I say can use, I dont mean it will Work, but Will Work well.


Thanks.


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60 Ranchero - Project ( Money Hole )
85 Sport Coupe LG4 - Daily Driver

Just another Hot Rod kid, or thats what they all tell me.
Livin' the Stereotype

 

Personally, I say swap it all out because it's what came with the donor car and was all designed to work with. It's the simplest way, and when you're finished, there's no question it's all matched unless someone before you did the same thing. I mean come on, if you're gonna do it, please do it right and don't do a half-arsed job. Maybe it's just me, but I think swapping the correct proportioning valve entails much less work than trial-and-error brake bias adjustment.

Biochem: You've obviously never rebuilt an abused automatic transmission. You wanna talk about stick-a$$ fluid...

 

Like JZA said.

JamesC

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It wouldn't hurt anything to try your existing master cylinder and p.v. first. If, however after the swap you have a brake bias issue (as I did) then you will need to sort through components to resolve it. The brake bias switch on the p.v. will tell you if you have gotten it correct or not. I suspect mine was caused by a leaking fitting at a brake caliper. But I also swapped my brake master cylinder. The weird thing is both were from a 90 RS and had the same A50 number cast on the body. I've attached a link to a prior discussion on this topic that may help clarify it. Good luck. Lon
https://www.thirdgen.org/messgboard/...ML/000456.html

 

Its always the 85s

i have no discs...

aprox how much will you end up spending at the yard and in brake lines?

maybe i should just keep the drums...

:P