Here’s a scary scenario we all dread as drivers. You’re cruising down the highway, wind flowing through your hair and “Born to be Wild” pumping from the stereo. Up ahead traffic begins to slow and you gently tap the brakes. To your horror, nothing happens! The pedal is stiff as a plank of wood. You have to practically stand on the brake to slow the vehicle down, but it does so eventually. Phew!
Let’s take a closer look at diagnosing the most frequent causes and solutions related to a hard brake pedal.
Vacuum Pressure
Vacuum – or really lack of vacuum pressure – is the most common cause of a hard brake pedal, and therefore the first thing to look at when a hard pedal is present. Any brake booster (whether from Master Power or any other supplier) needs a vacuum source to operate. In gasoline-powered cars, the engine provides a partial vacuum suitable for the brakes’ power booster. The booster requires 18” of vacuum to operate at full efficiency. Without the proper vacuum level, a brake booster will get a progressively harder pedal and eventually end up at a point where you feel like you are pushing against a wall. Your brake system’s booster works by a series of diaphragms inside the booster and air on both sides of the diaphragm. An improper amount of vacuum creates a scenario where the diaphragms can’t move the pushrod into the master cylinder. When this happens, the pedal gets harder. If sufficient vacuum isn’t being supplied within the booster, you may have to consider installing an electric vacuum pump, or canister depending on how far below 18-inches the vacuum pressure has dropped. An external vacuum pump is basically an electric motor built to provide vacuum to the booster that your engine can’t provide. It plumbs into the brake system using a vacuum hose going from the booster directly to the pump. This completely removes the engine from the equation and provides the proper vacuum level to the system. Before jumping directly to a vacuum pump though, there are a couple of quick and simple things that should be investigated. You should look at things like the hose supplying the booster from the engine. The most common problem we see is a person will be using a 3/8” fuel hose. Fuel hose is designed to resist expansion but won’t resist sucking closed like vacuum hose will. The proper hose to ask for is 11/32” vacuum hose. If you are running a fuel hose, when the engine is running and pulling vacuum on the booster, there is a good chance that the hose is sucking shut. If it is sucking shut, there is no chance of a vacuum being pulled on the unit. An obstruction in the hose could also be limiting vacuum contributing to the brake problem, so be sure to check this area thoroughly. Another quick check would be the location of the vacuum source within the engine and the fittings used to install the vacuum hose. We have seem many instances where people will use a port that is way too small, not allowing the engine to pull the proper vacuum through the fittings. Make sure you are using a port in the intake manifold that is no smaller than a 3/8” NPT. If all of those things check out within the system, another thing to look at is the actual size of the vacuum booster. Not enough assist within the booster can definitely cause a problem. A brake booster must be properly sized to the automobile that it is installed on. If the booster isn’t of the proper size, proper assist can’t be provided and the pedal will become hard due to the fact that the system is tapped out. At this point, the pedal becomes hard as the booster has done all it can but the vehicle still needs more. This can be the scariest of all scenarios when driving a vehicle. The vacuum present in the booster is the first, and most obvious problem to consider. Brake boosters require a minimum 18-inch vacuum to operate optimally. The further you dip below this the harder the brake pedal becomes. Following our advice above should set you on the right path, but as always, we are here to get you the right equipment if you need it.Valving Issues
Another culprit responsible for a hard pedal could be the combination valve, and in particular the Pressure Differential Valve within that valve. This valve is there for safety reasons but can cause headaches if things aren’t working properly. The Pressure Differential Valve is designed to move should pressure drop on the front side of the valve versus the rear side and vice versa. When this happens, fluid coming from the master cylinder basically hits a wall. Because the brake fluid can’t be compressed, the pedal becomes rock hard. You still can generate moderate pressure on the “good” side of the valve therefore allowing the car to be driven in a limp home scenario. This condition is what is commonly known as and referred to as a tripped valve. If your pressure differential valve has been “tripped” it must then be re-centered by equalizing pressure on both sides of the valve. This is accomplished by getting the valve to move the proper direction and therefore putting the valve back in the center. Of course, the problem that caused the valve to “trip” must also be corrected at this time. If your vehicle has OEM disk brakes it is highly likely that it also utilizes some form of valving within the brake system. In this case, the same valve that operates the warning light on your dash – the pressure differential switch – could be the problem creating a hard brake pedal. Follow our plan above to on the right path, but as always, be cautious and employ a professional if you are unsure. Properly operating brakes are essential for safety. We are always here to get you the right equipment or additional help if you need it.Pedal Ratio
Previously in this series, we addressed a hard brake caused by a tripped pressure differential valve, as well as insufficient vacuum pressure creating a too-hard brake pedal. Here we take up pedal ratio – essentially the relationship between your brake pedal length and where it pivots – an issue that comes to light with some regularity when drivers experience a hard brake.
Pedal ratio is overlooked by most people as a potential root cause of a stiff pedal. This is less of a problem with later muscle cars and more of a problem in earlier street rods when the booster/master is mounted under the vehicle. However, pedal ratio can be as big a problem in either case, so it must be considered as a potential cause for a hard brake pedal.
Pedal ratio refers to the relationship between the pedal’s pivot points and the length of the brake pedal. The pedal is used as a lever to apply motion to the booster (or directly to the master cylinder if your car does not have a power booster) based on the length of the pedal. If the pedal ratio is incorrect by as little as 1/4”, this can allow too little pushrod to move through to the booster. This, in turn prevents the booster from moving the piston into the master cylinder. The hard pedal you are feeling is actually the bottoming out of the pedal and its movement but leaving stroke within the master cylinder and therefore brake pressure at the wheels.
Correcting the pedal ratio can be sometimes difficult if it means moving the pedal pivot. The corrective action though can sometimes be as simple as relocating the connection point of the pushrod between the pedal and the booster. For reference, a power system should have a pedal ratio of 4:1 while a manual brake system should be 6:1.
Pedal ratio is not one of the more obvious causes of hard brake pedals. This is particularly true if the brackets and pedals are all factory installs. However, in cases where modifications have been made, this definitely may be an area worth looking into. Unfortunately during some brake modification processes, pedal ratio is not taken into consideration. After having new brake components installed you will need to reconfigure the pedal ratio to ensure optimum braking performance. Use the tips above or consult a professional mechanic.
What would cause a Brake pedal on my 1940 Ford slowly get hard and have no free play stuck on the top, can't push down and lock up the back brakes?
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