power DRUM brake conversion (with a dual master cylinder)

[jocko]

Good point fleet - after all the yammering (mine), it might be worth while to list the (my) lessons learned...

If you want to convert from non-power single reservoir master cylinder DRUM/DRUM (i.e. stock for most cases) to Power Boosted dual reservoir master cylinder DRUM/DRUM, here's the gouge:

1) Recommend buy the booster and master cylinder TOGETHER (or take them from another vehicle "together" so you know you have a compatible system) for a "Power Drum/Drum" setup. (ensures compatibility. I found that MOST vendors (oreily, autozone, etc) do not list a separate part number for power vs manual master cylinder by itself. My mistake was to buy a 67 C10 DRUM/DRUM master cylinder and 75 impala booster separtately - turns out the master is a MANUAL DRUM/DRUM master and you need a POWER DRUM/DRUM master. The way to tell is that a MANUAL DRUM/DRUM master will have a deep hole in the master's piston to accept about 1.5+" of the brake pedal pushrod. A POWER DRUM/DRUM master will have a very shallow indent in the back of the master's piston to properly mate up to a booster. Specific recommentation - buy CaptFab's bracket and a 67 C10 master/booster combo. All you'll need to do is open the holes on the bracket very slightly. IF you can find a POWER drum/drum master for a 67 C10 for example BY ITSELF - then good on you, I could not and the parts store zit squad wasn't any help either. Unless it comes puking out of their computer, they barely even know their own names (my apologies to any member counter jockeys - if you're on this forum, you obviously don't fit that category and I wish you worked at my Oreily's...)

2) a DRUM/DRUM master cyl will have equal size reservoirs - not as common as the uneven reservoir'ed disk/drum masters.

3) Confirm you have at least 15-16" vacuum before going down this road. Much less and I'd just get manual brakes so you needn't worry about it.

4) Confirm you actually have a fitting to hook power brakes to on the intake manifold. Or at least the plugged vacuum port so you can add the fitting. It SHOULD be located right over the #8 intake runner on the manifold in front of and slightly to the passenger side of the distributor. If you don't have one, some have said you can tee in to the pvc line from under the carb. In my case that no workie because I'm going to a dual quad setup and I don't think it'd fit even if I did tap the un-drilled boss on the manifold.

5) pedal ratio - For MANUAL: 6to1 / For POWER: 4to1. Talkin mechanical advantage here - i.e. pedal ratio = (distance from pedal pivot to the point on the pedal where the pushrod is connected) / (overall distance from the pivot to the center of the brake pedal). So, in other words, even if you do everything correctly in converting from manual to power, the basic brake pedal setup of your old manual brakes is technically incorrect. Personally I would not change the pedal ratio, messy business that - just be aware of it and realize you probably have close to 6:1 vice 4:1 so transitioning to power brakes will REALLY seem effective because you will still have the manual brake mechanical advantage (which is greater than what it should be for power brakes).

6) In most cases, you'll need to modify the connection between the booster and the current brake pedal push rod. Most boosters you buy will have an eyelet on the end where it WOULD have connected to a bell crank mechanism. Hack it off and make a home-made connection to your existing brake pedal pushrod. Helps to design a little adjustability into the design so you can tweak it.

I'm sure I'm missing something. But that's obviously nothing new....