Welcome. Anything uniquely Packard will usually have to come from one of the vendors. Kanter Auto
http://www.kanter.com/packard/pac-556.html or Max Merritt
http://www.maxmerrittauto.com/ are a couple of the larger ones and either should be able to help you with parts, kits or a rebuild of the brake unit. It is a Bendix Treadlevac and other places also do them. Being as it is a bit of a controversial unit, I would recommend strongly you do have it looked at professionally by a reputable rebuilder. There are a lot of threads here about failures of the unit or BTV as it is usually called if you want to search for them. Some failures can be blamed on improper rebuilds or not changing something marginal.
There are also a few modern cross references for common parts like hoses and cylinders other cars may have used listed in the parts x-ref.
https://packardinfo.com/xoops/html/modules/parts/index.php?cat=7&catname=BRAKES While most of the xref are good numbers, sometimes there is a bad or obsolete one that slips thru so a comparison may be needed.
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