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Tech Talk with Master Power Brakes

Troubleshooting a Hard Brake Pedal in a Power Booster Equipped Vehicle

Posted by mpbrakes1 on Sep 2, 2015 4:32:37 AM

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!

This post on vacuum pressure is the first of our 4-part series in which we take a deep dive into 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.

Topics: Brake Troubleshooting, brake vacuum, hard brake pedal, power booster

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