Box Rear Axle, Disc Rear Swap

[sxcpotatoes]

I've searched and cannot find any instructions here on what brake system modifications need to be done to swap a drum rear on a box to a '92-97 disc rear end.

The '82 Mark VI is in the transmission shop for the rear end now, and when they tore into it, it is beyond hosed.

Wear on the axles, chewed up where the bearings are. Spider gears are shredding, chunky. Posi unit is toasted, clutches destroyed. About the only thing salvagable are the 3.73--that's right, 3.73 gears. When it was supposed to have 4.10s in it. On a rear end that was almost COMPLETELY rebuilt 2 years ago, and not driven much.

I want everyone to know I don't blame Sly at all for this, but I am going to rip that shop a new arsehole or 7 on Google+ Pages for their business.

Anyway, for now, I'm going to grab a 66k mi 2.73 rear from a '97 Grand Marquis found locally, to have the shop swap in to gain disc brakes for the rear. I will either reuse the 3.73s or go with a ballsier gear down the road when I have a posi unit put in.

I did some searching but couldn't find anything at all for what modifications (other than the E-brake cable) need to be done to convert the box rear to discs. Is it just taking out the proportioning valve or what? Any help would be appreciated.

The plan is, the rear goes in tomorrow (fingers crossed no driveshaft attachment issues from '82 to '97), then it gets towed to my regular mechanic with easy instructions for him to modify the brakes to get everything working perfectly. Thanks!

[HiFiMerc]

I've heard some have had good luck with using the stock Box brake system with rear discs.

[BlackVic_P71]

I've also seen the use of '03+ rear brakes

[gadget73]

The stock box brake system works, but its not ideal. The prop valve and residual valves are not correct for disc/disc applications. The residual valve holds some pressure to keep the wheel cylinder pistons from folding in and leaking. With discs, this just means you wear pads more rapidly. If you can, swap the master cylinder out for a 1990s model that came with disc/disc. They have the prop valve screwed into the side, and no residual valve. I might swap the booster to the later one too, but its not absolutely needed. You will get some more assist from the later one. If its got the combo valve down on the frame, eliminate it. There should be one line from the prop valve screwed into the later MC back to the rear, and the line from the front should go from the MC down to a tee on the frame where it splits to the two front wheels. In the back, you have options, but I'd probably duplicate the disc brake rear plumbing. Use the non-ABS hoses. On the left side, the hose has what looks like a tee built in. The line from the MC screws into one side, and the other side feeds over to the right side brake hose. You'll need to weld a stud (aka a 1/4" bolt) or similar onto the frame to mount the hose to. Its got a tab with a mounting hole. I guess a self-tap screw would work but thats cheesy.


Honestly if the rear was that messed up, I'd wonder about those gears. I believe they are serialized. Make sure they match, otherwise its scrap metal. The only thing that comes to mind that would kill a rear like that is massive lack of lube. If a seal failed or they just didn't fill it up, that would do it. Normally what you get from improper setup is whining or it could chew the gear set up. None of that would affect the axles or outer bearings. No dino goo will do it quickly tho.


The early driveshaft yoke is smaller in the back. I believe you're going to have to swap the pinion flange from the 82 onto the later rear, unless it happens to have both sets of bolts already on the late rear. My car has that for the tow package rear, but I don't know if they kept with the 2 sets of holes or not. My car had a large flange driveshaft, but it currently wears a small one. The Mark VI has a specific driveshaft too, so you're not going to be able to just swap in a later one with the big rear yoke. The 82 should probably have been an 8.5" rear, which is effectively identical to an 8.8 except the ring gear is 1/4" smaller. All the rest of the stuff should interchange, including the flanges.

[mcninetyone]

I swapped a complete 92 rear into a 91 using all the 92 brake hoses and lines. I also use the 92 e-brake.
I found the frame already prepped for the disc brake rear.
But I didn'y swap the MC, booster or anything else, maybe I should have, but so far it's all working great.

I'm now getting ready to swap a 2000 whale rear into an 02, so I don't expect any problems, this one should be even easier.

[pantera77]

From the description of how bad the rear end was, I wouldn't reuse those gears.

[sly]

that would explain a lot of the higher speed issues I was having... AND the speedo issues (the speedo needs to be recalibrated as the speedo gear is correct for 3.73 - more suckage). This means that it was sold to me wrong (was always under the impression that the gears were 4.something) and since I'm not real up on drive train issues, I never verified all that crap. It worked, and the shop that I went to (which is no longer there... and apparently for this reason among others) never told me anything other than wheel bearings and an axle. I don't think they even did carrier bearings or pinion bearings. I'll have to defer to Nick for that as he has the records now.

[86VickyLX]

Lincolnmania and a few others had the rear disc set up with stock brake stuffs. Me included. I had no issues with rear brakes wearing down prematurely or anything like that. Stopped great and pedal was always at a good height. Only thing you gotta really modify is the parking brake cable. And just use rear hoses for an abs car, and just flare up your original rear lines to attach to them. That's what I did. Worked great.

[sxcpotatoes]

Wait, one of you said use non-abs hoses, other said use abs hoses, which is it?

Good news, everyone! It came with 2 sets of holes for driveshaft attachment AND rear swaybar already on it.

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[sly]

non-ABS for master cylinder/booster stuff if replacing that up front to match the braking system you're installing... The rear axle stuff, I would listen to John as his box is pretty awesome and actually has ABS system installed up front too. Just needs more power

[pantera77]

Rear sway bar is useless unless you weld some brackets on your frame to attach it too.

As everyone else said, you can run the disc with the stock box M/C and proportioning valve/splitter, but it really isn't ideal. 92-97 non abs master cylinder which has the proportioning valve that screws in the rear port, then rerunning the brake lines and deleting the factory proportion valve/splitter/magic box is the proper way to do it. Upgrade to a PI spec 92+ booster too if you want better pedal feel.

[gadget73]

The hoses depends on the rear and your plumbing. If you retain the orginal box rear and want to use the existing on-axle brake lines you need the ABS hoses and you have to mount them to the axle itself somewhere. if you change it to a late rear that does not have the steel tubes on the axle, you have to use non-ABS hoses and mount them to the frame like a round car has. If you're putting ABS on it, then you need a second line to the back.


If the rear is that shot, a later one makes more sense than a rebuild. They are also wider than the original by a small amount. Up to 1997 will bolt in.


rear roundcar bar would involve slight frame mods to allow it to mount. Not a huge issue, but I believe you'd need to have some plates welded in.

[gadget73]

aha, found it. Some light reading on panther brake system specs:

http://www.grandmarq.net/vb/showthread.php?33592-Brake-system-specifications

some further explanations on the valving. The stock box system uses all of the stuff described here. The rear disc setups use only a prop valve. No metering, residual, or differential valve is used or needed. The idiot light wires to a float on the master cylinder.

http://www.studebaker-info.org/Brakes/bf50014.html

[sxcpotatoes]

Thanks for all the help guys. The '97 rear (with 64 or 66k according to the yard) is in it and it's towed to my regular mechanic's as of yesterday before closing (he'll work it in around other stuff). Don't know how long the rear sat but rotors are rusted, it'll roll but calipers drag. Told Dave (mechanic) turn 'em or just do rotors & pads.

Going to have him take the rear swaybar off for now, and he said he was going to just hook up what hoses were there. I also picked up a '97 non-ABS non traction control MC at the same Salvage Yard, in case it needs changed in the future.

I just needed a working rear under it so I can get it oil/diesel bathed before salt and brine start polluting the roads. Something could happen to one of my other cars, and I could need to use the '82 until the other one theoretically gets fixed or replaced.

Plus, now I have a good idea of what's involved if I want rear discs on the '91. Also kept posi carrier, even if I toss the ring gear, I've got some CL ads w/ 3.55s, 3.73s, & 4.10s bookmarked right now.

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[Mr Bean]

Could always take the easy way out and swap to 03+ frame.