Brake Plumbing

By Dick Harrington

The Granada front disc brake and dual master cylinder swap are some of the most common discussions among us Cometeers. One of the most misunderstood ingredients of these swaps is distribution of the brake fluid out to each wheel. I have read and heard more misinformation on this subject than correct information. I will try to explain the three functions required for a safe and secure brake upgrade.

The valve that Ford places inline between the master cylinder and the rear-wheel cylinders and the front calipers has three functions: 1. fluid distribution (proportioning), 2. sensing pressure differential, and 3. maintaining residual pressure (metering valve). Lets examine each of these functions.

1. Disc brakes require more fluid pressure than drum brakes. The proportioning valve reduces fluid pressure to the rear drums to prevent premature rear wheel lock up. The lack of a proportioning valve or an incorrect reduction in rear fluid pressure rarely shows up on high traction road surfaces, but let the road get a little wet and or a heavy application of brake pressure will have you facing the wrong direction pretty quickly.

2. When dual master cylinders were mandated for safety reasons by the government, they also required a warning light to indicate if either system failed. The distribution block has a switch that when centered (equal pressure to front and rear wheels) is in the off position. If a pressure difference develops (because of a loss of fluid) the switch will slide towards the leaky side and send current to a warning light that indicates a failure in one side of your brake system.

3. Because drum brakes have shoes that are pulled back from the drums by heavy springs, they react slower than disc brakes when the brake pedal is pushed. To compensate for this, residual pressure is maintained to the rear brakes so that the front and rear brakes come on simultaneously. Lack of residual pressure is the opposite of no proportioning valve. In other words, the front wheels lock up before the rear wheels. Not as bad as rear wheel lock as far as directional stability, but the front brakes will wear excessively and stopping distances are not optimized.

Granada Proportioning Valve

Many of the Granada disc swap articles available on the Internet discuss using an adjustable proportioning valve. If adjusted correctly, you could prevent the rear from swapping ends. What is the proper adjustment? Are they being adjusted when the road surface is dry, or when it is wet? Anyone who saw Dale Jarrett swap ends and hit his jack man at Indianapolis in 2003 saw the results of too much rear brake in a panic stop. Yet Dale was very comfortable with the rear brake bias during racing conditions (moderate braking). A panic stop is not the time to find out that you don't know diddilly about adjusting a manual proportioning valve.

In my way of thinking, if you are going to use Granada disc brakes, also use the combination valve (proportioning/pressure differential/metering) that Ford designed for the Granada. The rear brakes are very similar between our Comets and the Granada/Monarch/Maverick.

View of Granada Proportioning Valve installed on a 1965 Mercury Comet Cyclone.

View of GEO Metro Power Booster, Master Cylinder and Granada Proportioning Valve in the background.