Boro -
The Treadle-Vac is actually a very simply and effective design. It doesn't sound like that spring is your problem.
Upon removal of the reservoir cover, you
should see the stem of the compensator port valve tipped at an angle. Again, I suggest you check out the 51-54 Packard Service Manual, which is available here, free, to download:
https://packardinfo.com/xoops/html/modules/news/article.php?storyid=4 At the very least you should download the Brakes section and study the details of the cross-sectional diagrams, but I'll walk you through it.
When the hydraulic plunger is retracted (by the BIG spring in the vacuum cylinder, when your foot is off the pedal), the washer on the end of the plunger tips the compensator port valve open, form the bottom, to permit brake fluid to move freely between the reservoir and the hydraulic chamber.
Whereas a conventional master cylinder moves fluid by pushing a seal down the cylinder chamber to reduce the volume of that chamber (like a piston in an engine), the Treadle-Vac merely applies the princicpal of displacement in a slightly different manner. Yet, you don't have to be Archimedes to undestand it. Fill your car wash bucket to the rim with water, then roll up your sleeves and put your fist in the bucket, all the way down to the bottom, and you'll see how this type of displacement works.
When you apply your foot to the brake pedal, the plunger moves down into the bore of the hydraulic chamber, and the washer moves away with it, releasing the bottom of the compensator port valve. With that valve closed, the hydraulic chamber is sealed off from the reservoir. As the piston moves further into the bore, the volume of the plunger that is introduced in the the hydraulic chamber will displace an equal amount of brake fluid.
Now, with the compensator port valve closed, and all other seals in working order, the brake fluid has nowhere else to go but into the hydraulic lines - ultimately pushing the shoes against the drums. However, if the compensator port valve spring fails to close the valve (or the seal on that valve goes bad), the displaced fluid goes back into the reservoir, which is vented to atmosphere - hence, shoes don't get applied tot he drums.
Get it? (I think you will.)
The real beauty of the TreadleVac design is that you should never need to have that master cylinder sleeved, like so many other cast-iron designs that are obsolete. Beware, however, of TreadleVac units that been salvaged from Packards that have been sitting in a field for a long time. Placed so low on the these cars, moisture from groundwater will take its toll on the die-cast housing. I have seen some pitted badly enough to leak. As such, I would highly recommend that, prior to reassembly, you coat the outside of the master cylinder housing from your car with a paint that is resistant to brake fluid. Check with suppliers like Eastwood for that.
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