A few weeks ago, a friend of mine called from back east to ask about a problem he was having with the brakes on his recently purchased 40 Ford Street Rod. The front disc brakes were dragging and wearing out quickly. The stock master cylinder was also leaking. Since I have a variety of repair parts at my disposal, I had him take it off the car and send the master to me.
Just as I suspected, I found the source of his front brake drag still in place inside the master cylinder. The residual valve. This little misunderstood valve can create havoc in a brake system both when used improperly, and when it is absent.
Very simply, the residual valve found in the drum brake systems is not compatible with the disc brake conversions you install to update a vehicle?s brake system. This valve, as with all valves of the brake system, have very specific guidelines governing their usage. The purpose of this article is to explain each valve and it's proper usage. Everyone with a modified brake system should check their system valves to be sure they meet the guidelines discussed here. If they do not, corrections should be made immediately.
The street rod mentioned here has very little, if anything in common with the kind of cars most of you reading this article drive, but the idea is the same. You can?t just make changes to your brake system without putting in some thought as to making sure the system will work when you are done.
Whether you drive a Honda Civic, Acura Integra, Nissan Ultima, or any other kind of car, the brake system consists of much more than just the brakes on the wheels. The system valves are meant to control how the front and rear brakes work together to give you a safe vehicle under all stopping conditions.
The whole purpose of installing any valve in a brake system is for one simple purpose. That is, to make the braking power being generated from the front and rear systems, consistent with each other throughout the pressure ranges encountered under all stopping conditions. If you graphed the brake torque from the front and rear brakes and they were in the proper distribution under all varying line pressures, there would be no need to install any additional valves. Take a look at the graphs included in this article and see how disc and drum brakes differ in their torque output, and how torque generated from each becomes unbalanced at certain extremes.
We may as well start with the residual valve, because it is the first one that should be determined whether or not it is needed. This valve does exactly as its name suggests. It keeps a pre-determined amount of residual pressure in the line after you remove your foot from the brake pedal. This aids in preventing excessive pedal travel as well as insuring consistent height to the pedal. In a drum brake, heavy return springs are present to pull the shoes away from the drums. When not in use, the shoes are pulled back until they rest on a centering pin, usually located at the 12:00, or top position, on the backing plate also holding the wheel cylinder. In order to avoid the excessive pedal travel to move enough fluid from the master to activate the shoes, a 10-12 pound residual valve is installed in the line. Sine the return springs are stronger than the 12 pound valve, the shoes are pulled away from the drum in spite of the resistance so no brake drag results.
A disc brake system, however, cannot tolerate this kind of pressure, as it would cause the pad to rub the rotor even when your foot was off the brake pedal. 10-12 pounds of line pressure on a disc brake will cause detrimental drag and a tremendous decrease in pad life. Worse yet, if the vehicle is driven at a consistent speed, the temperature will climb, due to this drag. This will cause the pads, rotors and brake fluid to swell causing lockup. At that point, the only way the brake system will release is for everything to cool back down.
In a stock system, the master cylinder is mounted high on the firewall. The gravity of the fluid in the master will cause 1-2 pounds of natural residual pressure, which is sufficient to maintain constant pedal height in a disc brake. It is for this reason, in most cases, a residual valve is not used with a disc brake. There is one very notable exception. This is when the master cylinder is installed lower than the caliper or drum wheel cylinder. In this case, the fluid would want to return to the master cylinder by flowing downhill like a river. The result would be a low pedal or even no pedal at all! To stop fluid rollback, we would want a 2 pound residual valve, which is just sufficient to stop rollback, but not enough to cause harmful brake application.
If you are running disc brakes with a master cylinder mounted below the floorboard without a residual valve, such as you might see on a highly customized car that would be trying to hide the ugly master on the firewall, excessive pedal travel and brake loss could result. Correct this condition immediately, even if you have not had a problem yet.
To really get the most practical benefit from this information, you first need to know what valves are in your system, and where they are located. The residual valve, for example, could be found in a variety of locations. On most American cars and trucks up to the mid 1960's, the valve will probably be found inside the master cylinder. In a single master cylinder, it will be located at the bottom of the bore and look like a little brimmed hat. In a tandem master, it will be in one or both outlet ports and look like a brass seat. On most all American and foreign newer cars, the residual valve is usually incorporated with other valves in a "combination" valve. (More about the other valves later). This combination valve can be located on a stock vehicle by simply following the line out of the master cylinder until you arrive at a junction block. This block usually has two lines going in from the master and two lines going out, one to the front brakes and one to the rear brakes. Most commonly, the combination valve is found on the firewall, within a foot of the master, just beneath the master, or on the left frame rail near the lower "A" arm.
If you have removed the combination valve, usually found under the master cylinder, then you have removed all of the valves on your system. It is important that you contact someone knowledgeable on your vehicle to put it back to a safe condition, and read the rest of this article to find out the problems you have created.
It is also possible that a line valve may have been installed pretty much anywhere in the lines to either the front or rear, or both. Since the valve is only slightly larger than a standard connector fitting, use care in looking for this valve. After market valves can be installed backwards, even though most valves are clearly marked with some kind of an indication as to which end of the valve should point to the master or the brakes. If a valve is installed backwards, it is like not having one installed at all. Be sure a line valve is installed correctly.
If you purchased the vehicle and the system was installed by the previous owner, make sure you check it. You must now be sure it is working. Since a residual valve may incorporate a diaphragm, the use of a sharp object such as a pin could puncture the diaphragm make the valve useless. In the case of the brake seat type in the outlet port of a tandem master cylinder, it may have been removed entirely, as they are only pressed in.
There is a simple though not foolproof method of checking to see if your system has residual pressure. With the brake system at rest, open a bleed screw on a wheel cylinder or caliper. If even a few drops of fluid come out, residual pressure is probably present. If the wheels are off the ground on a disc brake car, depress the brake pedal firmly and lift off. Attempt to turn the wheel just slightly to feel the amount of drag. Open the caliper bleed screw and close. If the wheel now turns more freely, residual pressure is probably present.
Occasionally, another problem in the brake system will be mistaken for a residual valve. This is when the master cylinder pushrod length has been improperly adjusted in such a way, as to not allow the piston to return all the way to the back of the bore. If the piston does not return all the way, the cup in front of the piston will not move back far enough to open the bleed hole. When the bleed hole is not open, residual line pressure is the result. Unfortunately, this type of residual pressure is not controllable and gets worse the harder the brakes are applied. Master cylinder pushrods always should have "free play" to insure that the piston can return all the way back when pressure is released.
As you can see, there is more information about valves than we can cover in one article. So, in order to do a thorough job, we will complete this article next issue. Between now and then, if this article made you squirm and you identify with any of the conditions we discussed, check your system and find out how your system is plumbed immediately. If your car has a pedal that goes close to the floor after sitting for some time, and then is fine after the first application, you definitely have some of the problems we discussed. If your pedal height never seems to be quite the same, or if it is always low and spongy, you need to correct it. I've driven a car that had the pedal go to the floor. Believe me, it is a feeling that you never want to experience yourself. I was lucky. I'm still here to help you fix yours.
