I have a ’36 Packard with vacuum boost cable brakes and it works well, as far as boost is concerned. The unit has some corrosion inside where the seal sat for decades so sitting at a stoplight it has a bit of a vacuum leak that leans out the mixture and causes a minor stumble at idle. It doesn’t effect the boost. They are a bit grabby when cold but quickly lose that.
My Pierce is still a work in process and I have only driven it in short spurts up and down the cul de sac. I can say that the Pierce brakes work very well and unlike most drum brake systems seem to work as well backing up as going forward.
I have been anxious for 30 years to see how they work on the road. The challenge for any mechanical brake system is to get them adjusted. They all have some degree of mechanical slop in them that is minimized the tighter you adjust them but some level is needed so the shoes aren’t dragging and overheating. Pulling to the left or right is the biggest concern, next is the delay from taking up the mechanical slop. The torque tube on the Pierce I think has the potential to be less prone to uneven pulling, but I don’t know that yet. Both Packard and Pierce are solid front axle cars with an interesting device called a kick shackle on one of the front spring shackles. It is there to reduce the tendency to develop nasty steering shimmy when “balloon” tires were introduced. Packard and Pierce have different designs for these. The kick shackle introduced a new problem, when the front brakes are applied the force gets fed back into the steering arm and the front brakes had to factor this out by tweaking the geometry of the front brakes to pull a bit harder on one side than the other.
There are so many variables that come into play comparing the system on any two cars. One of the main ones that throws things off is the brake lining characteristics. Original linings were relatively soft asbestos which is hard to find. Newer modern linings are often too hard and have very different characteristics which can be extremely grabby one minute and fading the next. I had my Pierce relined with newer material that is supposed to be relatively soft and a suitable replacement for asbestos, but I won’t be able to judge until mine is actually roadworthy. As of a few years ago it was still possible to get asbestos linings for mid ’30’s senior Packards (if I remember correctly Pierce used 16″ diameter and Packard 15″).
Another variable in the S-W system is the clutch that activates the brakes. It is basically a disc brake with a soft lining that is immersed in the transmission oil. When you push the brake pedal you are applying pressure to the clutch to apply the torque coming from the driveshaft to apply the brakes. The original clutch material is no longer available and units that have been rebuilt generally have a Kevlar material that has been found to be a suitable substitute. Although it is clearly suitable, it probably is not an exact match for friction characteristics, so throws another variable into the comparison. I was very lucky on mine, the original material was intact with no significant damage or wear, so should work per original – except for those non-asbestos brake linings.