don't have back brakes on 1990 Ford Thunderbird SC

Enthusiast 01be16a5a93688912f14755221fb15211b518d9a788cf01b455e03cc1bd585ac
my brake pedal is stiff as a board back brakes won't work checked accumalater motor plug and it's not getting any power.the motor is good .i checked it striaght to the battery.changed relay ,checked fuses and still nothing can you help.
(2) Answers
(2) Comments
Vehicle Application: 1990 Ford Thunderbird SC 3.8L

Customer Concern: The brake pedal is intermittently hard and feels like there is no assist.
Tests/Procedures: 1. Check the 40 amp ABS MTR fuse of the Power Distribution Box (PDB) for a blown, missing, or poorly connected fuse.

2. Check the 10 amp ANTI-LOCK fuse of the Primary Junction Block (PJB) for a blown, missing, or poorly connected fuse.

3. Disconnect the 5-wire antilock brake pressure switch on the ABS pump and motor assembly.

4. Install a jumper between the Pink/Light Blue (PK/LB) wire and the Grey (GY) wire. If the pump motor runs, replace the pressure switch.

5. If the pump motor does not run, check the circuits of the pump motor and pump motor relay for proper power input and output to determine if there is a circuit problem, defective pump motor, or defective relay.

A. The pump motor relay wiring consists of a Tan/Yellow (T/Y) wire which supplies hot-at-all times power from the ABS MTR fuse.

B. A Purple/Orange (P/O) wire supplies ignition fed voltage from the ANTI-LOCK fuse.

C. A Grey (GY) wire supplies a ground.

D. The PK/LB wire activates the relay when the pressure switch closes and grounds it.

E. The Grey/Red (GY/RD) wire carries power from the relay to the pump motor when the relay is activated.

F. The pump motor also has two GY wires that supply it ground.
Potential Causes: Defective ABS Pump Circuit
Defective ABS Pump Motor
Defective ABS Pump Motor Relay
Anti-Lock Brake System (ABS) Pressure Switch — Antilock pressure switch, circuit, or connection.
Antilock Brake System (ABS) Pressure Switch Circuit — Antilock pressure switch, circuit, or connection.
Antilock Brake System (ABS) Pressure Switch Connector — Antilock pressure switch, circuit, or connection.
Open Fuse

Sounds to me like this is the Brake Booster.
Check to see if there is a leak of fluid anywhere,which may be a sign of the pump itself.
But if you have brakes that are 'HARD' to put to the floor then it is most likely the brake booster.
These aren`t expensive to fix,just match the part number to factory specs so you don`t end up buying the wrong part.