Problems with an early anti-lock brake system

July 1, 2010 | By Richard Prince


I have a 1971 Lincoln Mark III with Kelsey-Hayes rear anti-lock brakes. The rear brake line passes through a "module" that also has a vacuum hose coming out the side. This “module” has developed a brake fluid leak and I can't find any parts or service information for it.

I would like to find the tech manual so I can understand how they work. I have advertised in the Lincoln and Continental Owners Club magazine with no luck. I have the factory manual but the antilock system isn't mentioned. Do you have any ideas?


A lot of people, including some very knowledgeable car enthusiasts, don’t realize that quite a bit of today’s sophisticated automotive technology traces its roots back decades.

For example, serious research on antilock braking systems goes back to the 1940s, when the military was looking for ways to more effectively stop heavy aircraft from skidding upon landing. GM had developed a functional but crude and somewhat unreliable antilock system by the mid-1950s. The Road Research Laboratories developed a practical automotive antilock brake system called Maxaret in 1958 in Great Britain. Maxaret was first installed in a production car, the Jensen FF sports sedan, in 1966.

In the late 1960s, Kelsey-Hayes developed its own version of antilock brakes and called it an “Auto-Linear” system. Lincoln installed this system, which acts on the rear brakes only, on its Lincoln Continental Mark III beginning in 1969.

All vehicles experience a shift in mass during braking. That weight transfer toward the front wheels effectively “presses” the tires to the road and discourages them from skidding.

At the same time, the rear wheels get “light” and this encourages them to skid. This is why rear wheels are more likely to skid and why to this day some antilock brake systems control only the braking action to the rear wheels.

The Kelsey-Hayes Auto-Linear system utilized sensors on the rear wheels. These sensors transmitted information about rear wheel slip to a transistorized computer mounted behind the glove compartment. As you would expect, this computer was extraordinarily crude by today’s standards. Based on input from the sensors, the computer modulates vacuum to the control valve on the brake line to the rear wheels.

On your car, it’s the control valve that’s leaking. You can simply remove the valve and install a junction in the line to bridge the gap that would be created by the absence of the valve.

Obviously, the antilock brake system wouldn’t function anymore but that would solve the leak problem. If you want to repair the leaking valve, you will need to be resourceful.

I could not find any listings or sources for the valve’s seals or internal parts. But once the valve is apart and it’s determined why it’s leaking, a solution can be worked out. A good machinist can make a suitable seal, sleeve damaged bores, and make any needed parts. White Post Restorations ( makes what’s necessary to restore obsolete parts and can very likely fix your leaky valve.