Yours is a BENDIX TREADLE-VAC SYSTEM common to most cars of that period.
In the 54 MERC SHOP MANUAL, is there a diagnostic sequence offered to isolate problem(s)? What is your manifold vacuum?
It seems a major concern is the MC being an aluminum casting and if the system is not service as scheduled, the brake fluid crystallizing.
Read this -
http://packardinfo.com/xoops/html/mo...=18986&forum=4
Rebuild Kits are available -
https://www.parts123.com/parts123/yb...~~~~~MONTCLAIR