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Rear Drum to Disc Upgrade

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    #1

    Rear Drum to Disc Upgrade

    I can see the need to upgrade the rear brakes on my 86 GM in the future. I couldn’t find a sticky for this upgrade. So here are some questions I have:

    The question is which way is a better way to go?
    What are some of the common problems with this upgrade?
    Does the master cylinder need to be upgraded? The Proportioning Valve?
    Which donor vehicles should I look for?


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    I'd rather be a failure at something I love than a success at something I hate.
    George Burns
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    #2
    Originally posted by BigT View Post
    I can see the need to upgrade the rear brakes on my 86 GM in the future. I couldn’t find a sticky for this upgrade. So here are some questions I have:

    The question is which way is a better way to go?
    1. What are some of the common problems with this upgrade?
    2. Does the master cylinder need to be upgraded? The Proportioning Valve?
    3. Which donor vehicles should I look for?
    1. The parking brake cables need some fabrication to attach your stock cable to the newer style. Grab as much of the parking brake cables from the donor car as you can. I grabbed the entire cable from the drivers front door back to each wheel.
    If you're planning on using the stock brake hose on the rear, you'll need the brake hoses from a car with ABS. The non abs lines are different.
    2. The Master cylinder should be replaced, but it's not necessary. I know of a few people including myself that run the stock master cylinder with no problems. Others complain about the rear brakes wearing out fast, but I haven't seen anything in my case that shows worn out brakes. Proportioning valve on the stock box sucks.
    3. 1992-1997 Crown Vics and Grand Marquis, 1991-1997 TownCars (non limo prep).

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      #3
      Are you looking to replace just the brakes, or the whole rear end? I did the whole rear end and the hardest part was the parking brake.
      89 CV LX 225/60 x 16 tires, CC819 rear springs, Front & rear sway bar, trans & PS cooler from 90 cop car. KYB shocks, F-150 on rear. Dual Exhaust w/ H pipe. Dark brown door panels, carpet, steering wheel, trim parts from a 87 Mer GM. Power front buckets from 96 Jeep Cherokee. LED'S front & rear. 3G Alt from a 97 Taurus wagon 3.0. Electric fan. Rear axle from a 97 PI 3.27 with disk brakes. Headlight relays.

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        #4
        it's a direct swap except the parking brake cables and the fluid lines, i was the first to do it in 2004 i still have my 86 master cyl and booster

        1986 lincoln towncar signature series. 5.0 HO with thumper performance ported e7 heads, 1.7 roller rockers, warm air intake, 65mm throttle body, 1/2" intake spacer, ported intakes, 3.73 rear with trac lock, 98-02 front brake conversion, 92-97 rear disc conversion, 1" rear swaybar, 1 3/16" front swaybar, 16" wheels and tires, loud ass stereo system, badass cb, best time to date 15.94 at 87 mph. lots of mods in the works 221.8 rwhp 278 rwt
        2006 Lincoln Town Car Signature. Stock for now
        1989 Ford F-250 4x4 much much more to come, sefi converted so far.
        1986 Toyota pickup with LSC wheels and 225/60/16 tires.
        2008 Hyundai Elantra future Revcon toad
        1987 TriBurner and 1986 Alaska stokers keeping me warm. (and some pesky oil heat)

        please be patient, rebuilding an empire!

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          #5
          Thanks for the info!

          I’m OK with changing out the whole rear end or not. Other than mine is an 86 axle and I’m not sure the gearing yet, it’s been good so far with no problems.

          I know police models have posi, are there other reasons to swap out the rear end instead of just the rear brake parts?


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          I'd rather be a failure at something I love than a success at something I hate.
          George Burns

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            #6
            this is a big myth... police models usually were open 2.73 or 3.08 for highway rundown.

            Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
            rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)

            Originally posted by gadget73
            ... and it should all work like magic and unicorns and stuff.

            Originally posted by dmccaig
            Overhead, some poor bastards are flying in airplanes.

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              #7
              In Jersey and some other northern states all PI had open rears to prevent the cars from fish-tailing in foul weather...but here in SC all the PI's I've come across have had traction lock units.

              On a side note, if using an aftermarket adjustable prop. valve, can I just bypass my existing prop valve with the rear brake line or do I have to gut it then use the adjustable prop valve after it(the two fluid circuits seperate in the prop/combination valve)? This will be my first rear disc conversion and I'm not trying to have rear brakes that lock up or drag.

              Every little bit of info helps somebody somewhere.

              Good luck to OP,
              Don
              '85 CV coupe- 351W, T5-Z, FAST Ez-Efi, shorty headers, 2.5" duals with knock off flowmasters, 2.5" Impala tails, seriously worked GT-40 irons, Comp 265DEH cam, 1.7rr's, Mallory HyFire 6A, Taylor ThunderVolt 50 10.4mm wires, 75mm t/b, 3G alt swap, 140mph PI speedo, PI rear sway bar, '00 PI booster/MC, 95-97 front spindles, '99 front hub bearings/brakes, '92-'94 front upper control arms/ball-joints, 3.73's with rebuilt traction-lok, '09 PI rear disc swap, '96 Mustang GT wheels with 235/55R17's.

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                #8
                Don't use 2 prop valves. It does really wonky things. The stock valving is intended for a disc/drum combo. You'd be better off using either a later model valve for disc/disc or aftermarket stuff intended for disc/disc. The master cylinder is functionally the same early to late. Same bore size, etc though after a point (89ish) they went to having the valve on the side of the MC and the lines I think are bubble flare instead of double flare. The later booster is dual diapragm for more surface area and assist though. Not sure but I think you can fit the early MC to late booster.

                The reason you may have accelerated rear brake pad wear is due to the stock valving. With drum brakes, its common to keep some low amount of pressure in the line to make sure the cups in the cylinder don't fold in. I think its 10 psi or so. Theres no need for this with disc brakes, and its not enough to really notice but its just enough to make the pads wear a bit quicker than they would otherwise. There is also a valve in the assembly that holds off front brake pressure until the rear brakes hit about 100 psi. This is to get the rear drum shoes out in contact with the drum so the fronts aren't doing all the work. This is also useless and undesirable with disc/disc setups.
                86 Lincoln Town Car (Galactica).
                5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

                91 Lincoln Mark VII LSC grandpa spec white and cranberry

                1984 Lincoln Continental TurboDiesel - rolls coal

                Originally posted by phayzer5
                I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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                  #9
                  Installing a factory type prop valve from a later model 4whl disc set-up may be easier, but it's ideal to gut the secondary side and install an aftermarket adjustable valve after it to dial in the rear brakes...especially if you were to do this on a wagon or tudor...basically anything with a different weight from the parts donor car will require more or less rear brake. Adjustability really is a beautiful thing.

                  Good luck,
                  Don
                  '85 CV coupe- 351W, T5-Z, FAST Ez-Efi, shorty headers, 2.5" duals with knock off flowmasters, 2.5" Impala tails, seriously worked GT-40 irons, Comp 265DEH cam, 1.7rr's, Mallory HyFire 6A, Taylor ThunderVolt 50 10.4mm wires, 75mm t/b, 3G alt swap, 140mph PI speedo, PI rear sway bar, '00 PI booster/MC, 95-97 front spindles, '99 front hub bearings/brakes, '92-'94 front upper control arms/ball-joints, 3.73's with rebuilt traction-lok, '09 PI rear disc swap, '96 Mustang GT wheels with 235/55R17's.

                  Comment

                    #10
                    Options and adjustments are always good have. Aftermarket valve may be the best way to go. Not to change the subject of this thread, I was wondering about stainless steel brakes hoses? So I started another:
                    Pros and cons of stainless steel brake hoses... http://www.grandmarq.net/vb/showthre...os-and-Cons-of


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                    I'd rather be a failure at something I love than a success at something I hate.
                    George Burns

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