Here is some notes on the '89 Mustang master cylinder that I installed on my '60 Falcon This is the diagram I referred to when I did the plumbing on the master cylinder. This diagram of the Ford master cylinder shows the ports are on the left side whereas a Mustang's are on the right. (Ranger Pickup truck is on the left also.) There is a small difference in the reservoir bowl also. See picture below.

I find this master cylinder makes a clean looking alternative than the cast iron MS and will bolt to the firewall using factory holes. I also used my original push rod and rubber boot. The master cylinder has metric fittings. I used 1(one) 3/16 X 12 adapter for the back line and 2(two) 3/16 X 10 for the front lines. I had to purchase a 20" pre fabricated hard brake line to run to the front of the master cylinder port to the proportioning valve and bent my rear brake line to the proportioning valve. I also had to bend the original front brake lines to their new locations. I replaced the rubber bumper on the pedal support with a late model '70's F-150 brake light switch.

A note on the metric adapters I went to a quite a few part house's to find an adapter that was made without loose threads. Some had a lot of "play" that was noticed when the hard lines was screwed half way on. This caused the connection to leak. If you run into problem find another adapter manufacture. This loose fit would not be noticed when the brake line is bottom out into the fitting, but it WILL leak!

Kinda funny, one place had the right size for one fitting but the other fitting would have a poor fit. When I went to another supplier the other fitting was junk. I ending up buying from two different auto part stores.

From the repair manual, The master cylinder is located under the hood, mounted to the power brake booster, and is best recognized by the large fluid reservoir on top. The removable plastic reservoir is partitioned to prevent total fluid loss in the event of a front or rear brake hydraulic system failure.
The master cylinder is designed for the “split system” mentioned earlier and has separate primary and secondary piston assemblies, the piston nearest the firewall being the primary piston, which applies hydraulic pressure to the front brakes.

Brake control valve
The brake control valve is located in the master cylinder between the brake lines and the master cylinder body on base production models. The control valve assembly contains a proportioning valve in an aluminum body. It also contains a pressure switch.
The proportioning valve regulates the hydraulic pressure in the rear brake system. It is located between the rear brake system’s inlet and outlet ports in the control valve. When the brake pedal is applied, the rear brake fluid pressure passes through the proportioning valve to the rear brake system until the valve’s split point is reached. Above its split point, the proportioning valve begins to reduce the hydraulic pressure to the rear brakes thereby balancing the braking condition between the front and rear brakes. This condition will prevent the rear wheel from locking up and the vehicle from skidding out of control.
The brake control valve is not serviceable—if a problem develops with the valve, it must be replaced as an assembly.