1990 Thunderbirds have a Anti-lock braking system that does not contain anything that might be considered a "hydro-boost".
That year vehicle braking system uses a all in one master cylinder, abs hydraulic pump, accumulator type of device. All the components for the ABS system, excluding the electronic controller, are contained within the Master Cylinder and tightly integrated with the entire braking system of the car.
Brake assist is provided by the ABS hydraulic pump. If the hydraulic pump is not operating, you will feel a very hard pedal, as you are getting no assist whatsoever. You should also get an ABS light, and possibly a BRAKE light. A hard pedal can mean the ABS pump motor has failed, or the pump motor relay has failed.
If you can describe your problem more, we can probably help you narrow down your problem much better than the Ford dealer did. In the end, you may need the entire master cylinder assembly to be replaced, but it also could simply be a bad relay.
The link that 38SCPROJECT provided is to Prior Remanufacturing. Thus far they have proven to be the cheapest source for rebuilt brake parts for the particular brake system you have.