Disc Vader - Let the Force (and Mopar Action) take
you from rear drums to high performance rear discs
Over the last two decades, we’ve covered many disc brake swaps, but we’ve avoided rear disc swaps. To be brutally frank, the stock rear drums work pretty well, and they’re light, cheap, and simple. So, there wasn’t a compelling need to swap. Second, the only factory rear disc setup available (from mid-’70s Imperials) had many drawbacks, including lack of parking brake cables for other body styles, porker-weight, hard-to-find rotors, and problems with adapting them to other axles.
In the last decade, however, lots has changed. CNC machining centers, plasma cutters, etc., have created a virtual explosion in aftermarket components, some of which are at least as well-conceived as the Mopar ’60s and ’70s stuff was. One place where the aftermarket has gone almost wild is in the area of brake swap stuff.
We looked at a few new front setups in our August 2005 issue; after much searching and spec-checking, we decided that our primary installation would be the one from Master Power Brakes. We especially liked this kit since it uses reasonably lightweight vented rotors (11.125" diameter x 0.98"’ thick), and the calipers that are sized right in the ballpark for correct FIR proportioning (2.00" diameter). The calipers have an integral parking brake setup, so no heavy mini-drum setup is needed. Plus, the Master Power kit comes complete: every nut, bolt, and washer is in there.
Master Power neatly solves the wheel bearing end play problem by simply mounting their caliper bracket on the outside of the axle bearing retainer plate. Then, a supplied spacer, exactly the same thickness as the stock drum brake support plate (which you toss), is installed. Voila! No change in wheel bearing end play. This is cool.
After driving approximately 1,000 miles with the Master Power kit installed, we can say with certainty that it was worthwhile. The main difference you’ll notice right away is improved pedal feel. Not necessarily firmness, but how much easier it is to feel when a wheel is about to lock. This is due, in no small part, to the fact that the discs are not self-energizing, as the drums were.
There are other perfectly viable rear disc swap kits out there; we’ll take an overview look at some of them so you can decide what’s right for you. All of these kits permit retention of the stock rear wheel bearings and bolt on to 8.75” or 9.75” Dana axles.
So, without further ado, let the 1/2-inch wrench twisting begin. You know the drill: we won’t repeat ourselves, the captioned photos will walk you through the whole shebang.
The Master Power kit is not only complete, but ours contained more parts than needed, to cover any oddball parking brake permutations. All parts are brand new. The rotors, which are basically stock (though redrilled) ’80s Firebird Trans-Am pieces, arrived with too small a center register to fit the stock axles. Master Power suggests turning down the axle’s hub; we, instead, decided to bore the rotors. This way, you still have a wheel pilot. Aida’s Precision Machine, 3260 Rt. 9W, Highland, NY 12528, (845) 691-8222 solved that dilemma for us quickly. Newer Master Power kits (ours was early stuff) have this already done.
Wrench-twisting begins by stripping off the drum setup and pulling the axles, this is just service manual nuts-’n’-bolts.
Once the drum assemblies are off, either cap the hydraulic hose or block the brake pedal down an inch or so, to prevent emptying the master cylinder.
We had given Aldo a stock axle to fit the rotor to, but once we got our Moser axles out, they didn’t fit into the rotors. They turned out to have a larger flange than stockers. Another trip to the machine shop. There was no way Master Power could have anticipated this.
6. One of our first curiosities was to see the weight difference. Our stock 10 x 2.5’" drum setup tipped the scales at 27.5 pounds. (7.) Much to our amazement, the disc setup is lighter-by over two pounds. All unsprung!
8. Assembly begins by reinstalling the axles, with some fresh grease added to the bearings. Master Power’s spacer (arrow) takes the place of the drum support plate. You can use etther the stock gaskets, or some gear-luberesistant RTY, to seal these.
9. Master Power supplies button-head capscrews to replace the housing studsrequired, theoretically, because of the extra thickness of the caliper brackets. We preferred the one-handed ease of assembly of studs, so we searched for longer ones, figuring that ones from an Imperial with rear discs would do the job. That didn’t work; they were superceded in the Mopar parts system to studs even shorter than stock 8-3/4’" parts. Luckily, we found that our stock studs were plenty long enough. Much ado about nothing.
10. A trial mock-up showed no rotor clearance or interference problems. (11.) But our Valiant’s oddball quadrashock rear suspension meant that we had to position the calipers one “notch” (set of bracket holes) further forward than ideal.
12. 14-inch wheels, at least these 1970 rail ones, do not fit. Master Power says that 14” Magnum 500s will clear. Virtually all 15s we tried were A-OK.
13. Again due to our quadrashocks (a), we had to get creative with the hose mounting bracket. We wound up hoseclamping it to the axle tube (b). This isn’t as bad as it looks, since the rear caliper moves very little, unlike up front. So there’s almost zero stress on the mounting point.
14. When installing the calipers and pins, be sure that the sleeves are slid inboard (circled). This is similar to the front calipers on most late-model Mopars.
15. Before bleeding or hooking up the parking brake cables, the calipers need a quick adjustment to pre-position the pistons. The MP instructions are clear; it only takes a few seconds.
16. Now the cables can be hooked up, and the parking brake cable slack adjusted as always. Then the usual bleeding procedure.
17. If your wheel studs are too short, install longer ones (ours were fine, even with our thick aluminum wheels.) Done! All that remains is a road test and proportioning adjustment (which is very important, see Mopar Action, August 2005, page 22, for details on this.)