You need a different master cylinder for this application

Question:

Last year I purchased a 1966 Mercury Comet Caliente convertible. It’s a restomodification project and the car already has disc brakes all around. It has 1968 Fairlane disc brakes up front and the rears are from a 1978 Granada.

I had to replace the vacuum booster and the master cylinder as both were leaking badly. I was advised to use a Lincoln Versailles master cylinder and proportioning valve. With the 302 engine in the car I didn’t want to try running new brake lines and didn’t know where to put the proportioning valve. So I replaced the master with a brand-new stock master cylinder.

Now I’m advised that I should remove the residual valve in the master cylinder and I’m getting a little crazy.

One more thing, do you have any idea where I can get rear parking brake cables to fit this 9” rear with Granada disc brakes? Thank you for any help you can give me.

Answer:

Under normal conditions there are two sets of circumstances that call for a residual pressure valve in a brake system.

A drum brake system needs a residual valve to maintain sufficient pressure in the lines at all times. Those heavy springs that help anchor the brake shoes are designed to pull the shoes away from the drums, which is necessary to prevent them from dragging. At the same time, however, residual pressure is needed to, in part, counteract the action of the springs and provide quicker response when the brake pedal is applied.

If the car has disc brakes up front and drums in the rear then the residual pressure valve should be in the rear line only. The other situation where a residual pressure valve is called for is when the master cylinder is mounted lower than the level of the brake calipers. The valve prevents gravity from taking its course and draining the fluid from the calipers back into the master cylinder.

You don’t specify the original application of the stock master cylinder you installed but if it’s stock for your Mercury or any other car that had drum brakes then it’s wrong for your car as it’s now configured with four-wheel disc brakes. Removing the residual valve from the master you installed is not the solution. The drum brake master is not set up to deliver enough line pressure for disc brakes. Change to a master cylinder that’s made for four-wheel discs.

As far as the park brake cable goes, I’d start with cables for a Granada and then shorten or lengthen them as required, but at least you’ll start with the correct end pieces to match the Granada brakes that are on the rear.