Solving Brake Problems
If you have poor brake performance, you might have a problem with your master cylinder. The first step in a situation like this is to inspect the brakes at all four wheels and look for leaks. If the brakes are good and you don’t see any leaks, then you might have a problem with your master cylinder.
The classic symptom of a bad master cylinder is that when you’re sitting at a stop, your brake pedal slowly sinks to the floor. You might even feel your vehicle creep forward at a stop. This usually means your master cylinder has failed. If that’s the case, replace it.
Once again, you can rebuild a master cylinder, but it might be more trouble than it’s worth. For that reason, I recommend you replace a master cylinder if you find it to be bad. One of the main things to remember when replacing a master cylinder is to bench-bleed it before you install it. Bench bleeding is the process of bleeding the air out of the master cylinder while it’s out of the vehicle. I usually put them in a vice and work the piston after I fill the reservoir. They also make special tools that do this for you and make things easier. Sometimes a new master cylinder will even come with the parts to bench-bleed.
The takeaway, however, is that you do this before installing the master cylinder on the vehicle. If you don’t, it might be very difficult to bleed the air out of the brake system after installation. Once you’ve installed your new master cylinder, it might be easier than you think to bleed out the system. I normally only have to bleed the system at the lines going to the master cylinder itself; I don’t often have to bleed all four wheels when replacing a master cylinder. Here’s a video with more details.
One last note about master cylinder replacement: You might find your master cylinder requires a push rod adjustment. Only do this if you absolutely have to. If you get this wrong, your brakes might not work properly or might lock up altogether. Follow the service procedure if you have to deal with adjusting the master cylinder pushrod.
This might not be as easy as it sounds. Some brake fluid leaks can be downright hard to find. Start with a good inspection of all the brake lines and fittings. If you don’t see any leaks there, remove the wheels and inspect the brakes themselves. Pay particular attention to the wheel cylinders. Pull back the dust boots on the wheel cylinders and look for fluid leakage. If you see any, replace the offending wheel cylinder(s). Lastly, check the master cylinder by holding your foot on the brake pedal. Don’t push too hard; it’s easier to diagnose with just a moderate amount of pressure. If the pedal slowly sinks to the floor, then you likely have an internal leak inside the master cylinder.
Be through in your search for leaks. Any leaks in the brake system are a problem and should be addressed immediately.
The brake booster, or power assist, is normally located behind the master cylinder. Many operate by using engine vacuum and a diaphragm to help assist in brake application.
If your brake booster fails, you often have a very hard brake pedal. You might still be able to stop the vehicle, but it will require a great deal of effort to do so. The first thing to check is the vacuum line going to the booster. Next, inspect the check valve that might be located in the vacuum line somewhere. Sometimes they’re part of the feed line and cannot be accessed externally. If this type of check valve goes bad, you replace the entire vacuum hose. This check valve allows vacuum to go to the booster, but not back to the engine. When brake boosters fail, it’s often a result of a master cylinder failure. Sometimes when a master cylinder fails, it leaks fluid into the brake booster. This can sometimes eat through the rubber diaphragm in the booster and cause it to fail. Other times, I’ve seen the return spring in the booster break and rip the diaphragm. Sometimes you can hear a hissing noise under the dash when a brake booster fails. This is the vacuum escaping. This vacuum loss can also cause engine performance issues. Either way, the fix is the same: Replace the brake booster.
Some vehicles don’t use a vacuum assist for the brakes. This might be due to space considerations or because the engine being used does not produce a lot of vacuum, such as with a diesel engine. These systems often use hydraulic pressure from the power steering system to assist with brake application. If you have difficult power steering or have a hard time applying the brakes, you might have an issue with your hydroboost unit. Don’t forget to inspect the lines as well as the power steering pump for failures. Here’s a video on the replacement of a hydroboost unit in a GM van.