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I am not an experienced technician on these old brake systems but, I have done some work with them. (I jut want you to be familiar with me to understand I want to help.) (Two years with an IHC truck shop, some 25 years ago.)
You brake system is call (by most) a Hydro-Vac brake system. If you want to know more about this brake system use that phrase to point you to relevant information.
The first thing I was always told about these brakes is to "bench bleed" the master cylinder (firewall mounted unit) before installation.
You must also be certain to bleed the air from the line at the booster too. Any air in this section will cause all kinds of strange operation.
I am assuming this is a gasoline engine or if you have a diesel, you must have a vacuum pump. Either case you need to have a strong vacuum to the booster to have "power" brakes. If not, they will be hard to push like when your car is off and you press the brake pedal.
So, to start this troubleshooting, verify no air is in the line from the master cylinder to the booster cylinder. Next verify you have good vacuum to the booster chamber.
Let me know what you have found and I will be glad to help as much as I can.
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