That is an interesting question and hopefully someone has a good answer. One thing the BTV didn't have a lot of was pedal travel so anything that happens happens fast. Here is a 53 commercial demonstrating the very small pedal "tip toe" movement between no brakes and a full stop.
https://www.youtube.com/watch?v=eMgriMYCtNc One thing I could theorize is the BTVs we used rely on springs and a large rubber diaphragm in the power piston to provide some feedback for pedal feel and how the poppet valves are worked. If the rubber diaphragm has hardened or poppet valves are not seating as tightly perhaps there is enough change that the vacuum is not applied to the power piston as abruptly as it was when new and the response is slower.
Apparently the sensitivity was even an issue new because Bendix developed a hydraulic reaction BTV as a version after the one we used. Instead of the rubber diaphragm and springs sensing vacuum application to provide the feedback, it has a rod mechanism thru the ram which directly feels hydraulic pressure and provides feedback to the power valving.
This Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=194664