The brake master cylinder went out on my trusty '92 Vic so I replaced it with a rebuilt unit. The car has ABS brakes. Swapping the master cylinder was simple enough, two studs secure it to the power booster, the brake rod/plunger are a male/female fit and just slip together. However, when finished, braking effort was greatly increased…
While the new master did not come with a gasket and I noticed no gasket on the old one (there may have been one but it got returned with the core) I postponed taking it apart until the car began stalling while waiting at a traffic light…I pulled the cylinder, made a gasket to fit between it and the booster unit and BINGO! both problems solved… There is vacuum at that joint and it needs a gasket to function properly…
IIRC there should be a seal between the vacuum chamber and the push rod. Therefore there should not be vacuum on the push rod side of the master cylinder. You might give a call to a Ford service shop and see if they can connect you with a factory engineer. Ask him/her the question you have posted.
I double checked some reference books and all the breakdown diagrams of power brake boosters show a piston rod vacuum seal. On further thought I realized that having full vacuum on the secondary piston seal of the master cylinder would cause all of the reservoir fluid for that side of the brake system to be sucked into the booster if the secondary seal ever got lifted by a piece of grit or otherwise failed. Yes, it does sound like the piston rod seal is leaking vacuum.
Let us know what you find out.
I believe that leaking master cylinders are known to destroy that seal. It’s possible that the booster may fail later due to the brake fluid it was exposed to, if the original master cylinder was replaced due to such a leak.