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My only beef with Treadle-Vac is there is no vent between the cylinder and the booster, so if the unit develops a leak the results are catastrophic.
On the BTV units of that period there is a vent called the relief port which connects the reservoir chamber to the space between the hydraulic and vacuum seals. In Packards case two things seem to have happened to block the vent on units used after 53. On the first model BTV Packard used, the vent port opening was lower in the reservoir and the Packard flat lid did not block the opening. Later versions extended the casting containing the vent port opening flush with the top of the reservoir edge.
It appears other mfgs using the later BTV had a stamped lid with a bulge or channel which was directly over the opening so air could be drawn in if there was a vacuum leak or fluid could escape if a hydraulic leak. For whatever reason Packard stuck with their original flat lid and in combination with the gasket used for the other lids, the port opening was well blocked.
At some point a BTV version must have been released with no vent. Maybe the hydraulic reaction version of the late 50s?? Packard never used that version but a no vent casting is the way the repro castings are being made today. That could also be the reason some rebuilders were apparently filling the relief port on the early units with silicone during their rebuilds -- in effect totally blocking it for all lids. When the port is blocked a leak at either seal means fluid is either forced by hydraulic pressure or sucked by vacuum into the vacuum side.
The flat cover and gasket can be used without issue if time is taken to slot or remove the gasket material in the area that goes over the solid part of the casting containing the port opening. That small clearance space gained between the opening and the lid allows fluid to escape or air to be drawn in even with Packards flat cover..
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