Manual to Power Drum Conversion

Jake Fleming

New Member
Apr 18, 2018
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Long Island, NY
Hi all,
I'm pretty new here but I thought I'd share a little info for others in the future. Theres LOTS said about upgrading to disks but when I went looking for info on simply upgrading my manual drums to power drums I found info a bit scarce. So, here's the procedure, clear and somewhat detailed. No pictures, its pretty easy, you don't really need pictures to "get it." It's so easy you don't really need a guide either but maybe knowing how it's done will reassure those who are considering doing this.

Master Cylinder, Backing Plate (with modifications), some of the nuts
Brake Pedal (thats a biggie, we'll address it), manifold vacuum fitting

So, with this in mind, you'll need to buy a brake booster (Cardone 5473520) and a Power Brake Pedal. My booster was $80 with free shipping, and I found the pedal through for about $100 after tax and shipping. Hardware etc brought the total to around $200. As always, if you're running a single pot master cylinder, please, just don't. My booster appeared different in person than it did in the photo but fit perfectly. The pedal situation: You just can't reuse the manual medal. For one the power pedal attaches differently. For two, the pivot point is moved to purposely reduce leverage. For three, it has a way shorter travel. If you somehow attached the manual pedal you'd have brakes so light you'd always accidentally lock them up, and the pedal travel would allow you to quickly tear and ruin everything in the booster and MC. So, bottom line, you need a new pedal.

So now, the steps:
1) Unbolt the master cylinder, and pull it off of the pedal pushrod. There are two bolts outside and two inside under the dash. The metal lines are strong enough to support it, and no fluid will leak.
2) Unfasten the two nuts a the top of the backing plate
3) The brake pedal is now free, pull it out, and wiggle it out past the steering column.
4) The backing plate is now free, if it hasn't fallen off, peel it free of the firewall.
5) Modifications: Use a hammer to pop the two studs out of the master cylinder. If your pedal didn't come with the bolt that attaches the pushrod, or a pedal pad, move those to your new pedal assembly. The backing plate will require some mods. For one, don't peel off those gaskets, they'll help with keeping noise out of the cabin. Your new pedal uses the top two holes on the backing plate, and two additional holes which you will drill, using the two existing top holes as a positioning key. The booster will also require two new holes, slightly lower than the existing lowest. Using your two new holes for the top studs on the booster, you'll see where the two bottom studs need holes. The large opening in the middle will need to be cut taller as the pushrod on the booster goes through the plate higher than the existing hole, I used a cutoff wheel. All of these holes are already in the firewall, only the plate needs these mods, or just buy the backing plate off the car you get your pedal from and skip the mods.
6) Assembly! Place the pedal on first, passing the two studs in the top through the uppermost firewall holes.
7) Push the backing plate onto the booster, making sure the bottom wide ring of the rubber boot gets sandwiched between the plate and the booster.
8) Install the plate and booster assembly on/through the firewall, making sure the two protruding studs from the pedal go through the top two holes in the backing plate.
9) Bolting the MC to the booster. For clearance reasons I found that I had to remove the two small bolts holding the ovular plate onto the rear of the MC. Removing these caused the seal to start pushing out, so I had to kind of hold the plate in place while I slid the MC on to the booster studs. The booster has just enough room recessed for that plate, and thus tightening the MC onto the booster pushes the seal back in, and the plate back tight to the MC.
9) Thread nuts onto the exposed studs above the MC, and tighten.
10) On the inside, thread nuts onto all four studs and tighten. This is an exercise in patience and required extensions, universal joints, and plain regular wrenches. It's just a bit tight with all those brackets and such in the way.
11) Bolt Pushrod to pedal arm, and attach wires to brake light switch
12) Vacuum: You will need to remove the existing, very small vacuum fitting in the manifold near the rear of the engine. It's bolt head is 11/16 and it should come out pretty easy. I replaced this with a fitting sold as a fuel line fitting at advance auto. Exact same threads but with a 3/8 hose barb. (Dorman 785-406)
13) run 3/8 ID hose from your new fitting to your booster. Your hard line that feeds your heater and/or AC controls is now unhooked, as it used to be connected to that tiny fitting. Your new booster has a smaller capped auxiliary vacuum barb. I connected the hard line for the heat controls to this barb, using an inch of slightly larger soft vacuum hose to adapt the size difference. This may or may not reduce the amount of vacuum boost you experience while braking due to small vacuum leaks in the complicated vacuum operated system under the dash, old seals in the switch, etc. The heat controls must be connected to manifold vacuum however, and even if you connected them elsewhere, I'm pretty sure the net effect would be the exact same small reduction in total vacuum. So I do not think attaching them to the barb on the booster will harm anything, but if it makes you uneasy go ahead and drill and tap a new manifold port. My 383 had only the one hole available in the intake manifold.

You're done! It sounds long but thats only because I was so detailed. It took me 3 hours, in a heat wave. I'm satisfied with the results. I will say that they're still drums so there's not much gain in stopping power however the force exerted to stop is obviously cut significantly. And the ease of ability to lock up the brakes/ panic stop is improved. The sensation is a bit odd at first especially if you're very used to manual brakes. It feels easy to press, yet slower to stop than disks. I found myself unhappy with the amount of pedal travel, which was remedied by checking/tightening my drum adjusters (the self adjusters almost never really work.) There was nothing wrong with my new setup, the shoes just had too much travel.
Chrysler and other OEMs used to sell factory accessory packages to install power brakes where they didn't come from the factory. Up until the later '60s, I believe.