1983 Ford Mustang Stalling

You need to replace the brake vacuum booster

I agree with CADIEMAN, here are the instructions on how to do so. Check out the images (below). Let us know if you need anything else.

1. Remove stoplight switch and slide booster pushrod, bushing and inner nylon washer from brake pedal pin.
2. Remove air cleaner.
3. On models with V8 engine, disconnect accelerator cable from carburetor. Remove screws securing accelerator cable bracket to engine and rotate bracket toward engine. Disconnect inlet hose of choke water cover and position aside.
4. Disconnect vacuum hose from power brake unit.
5. Disconnect hydraulic lines from master cylinder and cap open lines and ports.
6. Remove master cylinder.
7. From inside vehicle, remove power brake unit to dash panel attaching nuts.
8. On models with speed control, remove control amplifier which is mounted on the lower outboard booster stud and set aside.
9. On all models, work from engine compartment and move booster forward until booster studs clear the dash panel, then raise front of unit and remove from vehicle.

I would get a rebuilt booster is it much easier. Check out the images (below). Let us know if you need anything else.