Anyone convert their 80-81 to rear discs?
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RWD 19xx-1984 DeVille and Fleetwood,
1985-1996 Fleetwood and Brougham Forum Discussion, Anyone convert their 80-81 to rear discs? in Past Cadillac Vehicle Discussion; The 80-81 RWD cars were the last to use the 8.75" ring gear "P" axle. They came with big 12" ...
  1. #1
    jayoldschool's Avatar
    jayoldschool is online now GM RWD V8 addict
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    Anyone convert their 80-81 to rear discs?

    The 80-81 RWD cars were the last to use the 8.75" ring gear "P" axle. They came with big 12" drum brakes. Interestingly, this same axle was used in the 77-79 Fleetwood Brougham, and it came stock with rear disc brakes. Has anyone used the 77-79 brakes on the 80-81?

    A few questions come to mind:
    1. Do the 77-79 components (backing plates, caliper mounts, calipers, rotors) work on the 80/81 axle tube mounts?
    2. Does the complete 77-79 axle swap in? Same driveshaft flange?

    I am aware that some have used the 94-96 B axle from the SS or 9C1 with rear discs. This can bolt into the D body, but requires the pinion yoke to swapped for the D body flange for the driveshaft. I would prefer to stay with the D body 8.75 axle, and use factory parts to convert to discs.

    Anyone have any experience/ideas/thoughts? Or, some 77-79 parts for me

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  3. #2
    sven914's Avatar
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    Re: Anyone convert their 80-81 to rear discs?

    I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be), and the wheel base was 121.5 (yours is 121.4), so the parts should be interchangeable.

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    Aron9000's Avatar
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    Re: Anyone convert their 80-81 to rear discs?

    Quote Originally Posted by sven914 View Post
    I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be), and the wheel base was 121.5 (yours is 121.4), so the parts should be interchangeable.
    77-79 would be compatible with his year. 1976 was the last year for the really big body, before they shrunk them down.

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    jayoldschool's Avatar
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    Re: Anyone convert their 80-81 to rear discs?

    I'm not sure, but I believe the 76-79 Fleetwood Brougham was equipped with a THM200 transmission (as should yours be),
    Nope, they have the big TH400, as does my 81 (which is the last year for it, along with the last year for the Cadillac big block).

    Now, any thoughts on the brakes?

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    cadillac_al is offline Cadillac Owners Connoisseur
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    Re: Anyone convert their 80-81 to rear discs?

    I believe you are correct that the 77-79 Fleetwood brakes would bolt on your axle. There are also several aftermarket companies making disc brake conversions for just about any old GM car. I don't recall any prices right now but I think they are getting cheaper.

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    jayoldschool's Avatar
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    Re: Anyone convert their 80-81 to rear discs?

    scarebird.com is the best place for rear disc conversions. Their brackets are only about $125 for most applications, then you use factory calipers and rotors to complete the install. They don't list an application for the 77-81 "P" (Cadillac) axle, however. I'll send them an email.

    I guess I will get scrounging the yards in the spring. Probably the best bet is to grab a complete axle from a 77-79 Brougham, then rob it for parts to convert mine. If the parts don't fit, I can rebuild the complete axle and swap it in.

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    472's Avatar
    472
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    Re: Anyone convert their 80-81 to rear discs?

    You can easily change to disc by using pre-1980 SeVille brackets (can change nearly any GM axle this way - I did it on my '83 DeVille's 8.875" rear, and my '69 Nova's 8.5").

    Cad rear;


    The 77-79 axle will swap in FYI.

    Attachment is Nova rear;
    Attached Images

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    Re: Anyone convert their 80-81 to rear discs?

    You can easily change to disc by using pre-1980 SeVille brackets (can change nearly any GM axle this way - I did it on my '83 DeVille's 8.875" rear, and my '69 Nova's 8.5").
    Great info, thanks! I will add the Seville to my scrounging. Hmmm, I will have to get two sets so I can do my 71 Pontiac (8.5" Chev 10 bolt) too! Are the brackets available new/repro anywhere, or strictly junkyard? Do you know if they are the same parts as on the 77-79 Brougham?

    A few tech questions:
    1. Any machining? (eg: turn down the axle flange to fit inside the rotor hat)
    2. Use the Seville rotors and calipers?
    3. E-brake hook up? Any mods?

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    Re: Anyone convert their 80-81 to rear discs?

    Do you know if they are the same parts as on the 77-79 Brougham?

    A few tech questions:
    1. Any machining? (eg: turn down the axle flange to fit inside the rotor hat)
    2. Use the Seville rotors and calipers?
    3. E-brake hook up? Any mods?
    I'm not sure about stock disc 77-79 cars - never seen one...

    1. Yes, actually - that's exactly what needed to happen (only the Cad rear though).

    2. I used SeVille rotors on the Cad, 4WD S10 front rotors for the Chev pattern. For calipers I used front calipers, which needs to change. They are too large requiring too much pressure (fluid) to work (mushy pedal feel always), so I will be going with a smaller rear caliper in the future. If you choose to use the bigger ones, be sure the caliper moves freely, I had to file a tiny bit to let them move, and use two passenger side fronts - this keeps the bleeder right side up (notice the SeVille setup puts one caliper in front of the axle, and one behind it). I installed a C3 Vette master cylinder and Wilwood adjustable proportioning valve to compensate, but still not thrilled with the pedal feel.

    3. By using front calipers the e-brake was omitted.

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    Re: Anyone convert their 80-81 to rear discs?

    Found some more info:

    GM had six different rear disc-brake caliper setups from the late 1970s to early 1980s. Most were for Cadillacs, but some were also used by Buick, Pontiac and Olds.

    1977-1978 Seville rotors were 5 x 5 inches on an 11 3/16-inch rotor. Axles were 1.4-inch C-clip type.

    The 1977 and 1978 De Ville, Riviera and Fleetwood used different calipers and mounting brackets, but the axles and rotors were the same as the above.

    The 1979 Seville and the 1979-1981 Firebird with the WS-6 performance package used a similar configuration to the assemblies listed above, although they used a slightly different caliper. The Seville rotors had the 5 X 5-inch pattern and 11 1/2-inch diameter, whereas the Firebird rotors were a 5 x 4 3/4-inch pattern, but had the same outside diameter. Camaros did not offer the WS-6 package on any of their models, although it will bolt on directly to 1970-1981 Camaros. This is the most popular of the GM disc brake swaps, as it fits all 10-bolt and 12-bolt rear ends, as well as the S-10 trucks and Blazers. All of these rears used the four-bolt axle flange measuring 3 1/8 inches across the top and 2 5/8 inches across the bottom. Pay special attention when performing this exchange on the GM 12-bolt axles, because the Seville rotors will not fit without having to machine the center hub of the rotor to fit the axle. The Pontiac WS-6 rotors will not have to be machined to fit the axle.

    All 1979 Fleetwood, De Ville and full-size Buicks also used the same axles and rotors as the 1979 Seville; however, the calipers are again different (as well as the caliper mounting bracket) from the above three.

    1976-1978 Eldorado rotors were 5 x 5 inches on an 11-1/8-inch rotor. Axle used tapered inner and outer bearings and cups, and 1 3/8-inch and 3/4-inch tapered axle shaft similar in design to a front spindle.

    1979-1985 Eldorado, Toronado, and Riviera; and 1980-1985 Seville rotors were 5 x 4 3/4-inch on a 10 1/2-inch rotor. Rear axles were plain steel beam, using a bearing hub assembly bolted onto the end in front-wheel-drive applications.

    Because the GM corporate axles are held in by C-clips, it is not necessary to swap these out. However, the axles will have to be removed, both from the donor car and your vehicle, to both remove and install the brackets and backing plates.

    One thing you may discover when searching for these parts on GM vehicles is that the left caliper is mounted to the front of the axle, and the passenger-side caliper is mounted to the rear of the axle. This was common on the earliest GM configurations, and does not mean someone has already done a changeover on that vehicle.

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    Re: Anyone convert their 80-81 to rear discs?

    Scrounging has finally paid off! Lookee what I just brought home:



    It's a complete 1977 Seville disc brake rear end. Of course, this won't bolt into my 81 (the Seville used leaf springs), but I believe (from my research above, and the advice from 472) that the parts will work! So, I will tear it down for the backing plates and calipers. I'll use new rotors, and probably use the calipers for cores (apparently the core charge is really expensive, so I'm glad I got calipers). These calipers have parking brake feature built in, so it will be nice to retain that after the swap. Once I have everything apart I will be able to confirm that the backing plates will work on the big Cad axle (if not, I'll just re-drill them!).

    Best part about this (besides having factory Cadillac disc brakes showing through my wire wheels)? I paid 40 bucks for the complete axle

  13. #12
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    Re: Anyone convert their 80-81 to rear discs?

    Am I correct in saying that you don't have to use a proportioning valve when you install disk rear brakes?

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    Re: Anyone convert their 80-81 to rear discs?

    No, you still need a proportioning valve.

    I am considering using the master cylinder from the 77-79 FWB that had factory rear disc brakes. It is different than the MC that came on the drum brake cars. We'll see what the pedal feel/effort is like after I get it completed. The good thing is that I have the correct rear calipers. People that have used front calipers on the rear report mushy pedal feel due to the volume of the caliper being too high (and they wouldn't have a parking brake).

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    csbuckn is online now Cadillac Owners Master
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    Re: Anyone convert their 80-81 to rear discs?

    nice find
    is the parking brake gonna bolt up or will it be changed?

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    Re: Anyone convert their 80-81 to rear discs?

    Parking brake will bolt up (may need mods, of course...), and must be used.

    I have found out that the reason people used to have problems with the Cadillac rear discs is because they DIDN'T use the parking brake. These calipers require the use of the parking brake every time the car is stopped. This causes the piston to ratchet the pads down and maintain contact with the rotor, and when released, the piston stays extended. This keeps the piston out as the pads wear. If you don't use the parking brake, the brakes will be soft, and will require an extra pump to build pressure. I have also found all the info on how to rebuild them, so I will do that myself instead of trading them for rebuilds.

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