Brakes: Part 3 (Hydraulic inspection)

So, hopefully, your technician has taken the time to thoroughly inspect the components in Part 2 and has taken all appropriate measurements. Next, on the visual inspection tour, is hydraulics. There are several hydraulic components that make up your brake system. If you were to follow a flow-chart from your foot to the wheels it would look like this: the brake pedal pushes on the brake booster, the booster on the master cylinder, and master to the calipers and wheel cylinders at the wheels. So, lets start from the top and I’ll give you a general rundown of each component that our repair shop looks at.

The brake booster is normally mounted on the firewall inside the engine compartment in front of the steering wheel (its big and round). It uses vacuum from the engine to take the pressure of your foot on the brake pedal and multiply the force. A piston pushes into the master cylinder and forces brake fluid down the brake lines. In general, you will know the booster is bad because it no longer multiplies your foot’s pressure. It will feel real hard and stopping will be difficult. You will know the booster is going bad if you hear a “whooshing” sound when you press the brake, like air escaping. The brake pedal may still feel okay, but it is just a matter of time.

The master cylinder is mounted to the booster and it will have a cap on it where you fill the brake fluid. The technician should inspect the connections at the booster and at the hoses for any leaks. The master is really just a simple piston used to force brake fluid into the brake hoses, but its design is fairly complex. There are a series of seals and cut-outs that move brake fluid back and forth. Over time, these seals degenerate and allow brake fluid to seep past. One of the signs that this may be happening is a low or spongy pedal. It may go all the way to the floor, or sink slowly as you apply pressure. However, don’t let your mechanic condemn the master cylinder without first isolating it. With all of the brake hoses clamped off so that the brake fluid cannot move, the mechanic will step on the pedal. If it is firm and does not sink, then the master is in good shape (see note below on hose clamps). A final note about master cylinders: if it has the snap-on type of lid (vs. screw on) it will have a rubber gasket on the inside of the lid. Most of these lids have a very small opening at one end acting as a vent to allow air pressure to escape. If the gasket is put on backwards it will cover this hole. In addition, the lid needs to be installed so that the hole is on the high side (since air rises).

Calipers are the hydraulic component of disc brakes. When you step on the pedal, brake fluid is forced into the caliper which forces a piston to extend and push on the brake pad. When you let go of the pedal, the fluid returns and the piston should retract. External leaks are easy to see. However, an internal leak in the piston seal may not become evident until the caliper is actually removed and put back in place. To remove the caliper, the piston needs to be forced into the caliper. This is all fine and dandy. However, after the pads and caliper are reinstalled, the brake pedal needs to be depressed to force the piston back out. You may not know there is a problem with the caliper until this point. I hate when this happens because it now becomes an unforeseen expense to my client. There are two other visible signs that a caliper may need to be replaced even before starting. 1) If the pads are very low, especially metal to metal, the piston may have become overextended, preventing the piston from even being forced back into the caliper. 2) If one pad is a different thickness than the other pad this could indicate that the piston is not moving properly to allow both pads to create contact. 3) Signs of overheating on the rotor or pad (glazing) could also indicate damage to the piston. I have seen pistons warp from heat, thus creating problems when pushing it in or out. A final note on calipers, the pins have to be lubricated. This allows the caliper to float freely so that both pads make contact with the rotor. Stuck pins are the most common cause of the uneven pad wear described above. Be aware that the pins and their protective boots may need to be replaced if they are bent or torn.

Wheel cylinders are the hydraulic component of drum brakes. It looks like a small cylinder with rubber boots on either end. The brake fluid forces a piston on either side to push the brake shoes against the drum. A good technician will always peel the boot back to see if the cylinder is leaking. It may be holding fluid that has not leaked out yet.

A few words about brake fluid. First of all, it is highly corrosive. Don’t get it on your paint. Also, make sure that your mechanic does not have your expensive rims laying directly underneath the brakes, allowing brake fluid to leak on them. Now, brake fluid is designed to do a couple of things. Number one, it has a high boiling point, capable of taking the heat of hot brakes. Second, it is agroscopic – a fancy term meaning that it absorbs water. Water, or moisture, is the enemy of rubber seals, hoses, and gaskets in your brake system. Any water will immediately cause it to swell. As brake fluid begins to absorb water, it turns darker in color until it no longer does its job. A fluid flush should be standard on a brake job and is also on your factory scheduled maintenance list. [Side note: we had a client accidentally put coolant in his master cylinder. His brake system began leaking within a day. He had to replace his brake booster, master cylinder, both calipers, both wheel cylinders, and all rubber brake lines. Very, very expensive mistake.] So, back to the spongy pedal. Isaac Newton discovered that although air could be compressed, a liquid could not. If there is any air in the brake system, it will compress, causing a spongy pedal. That is why a technician should be thorough when diagnosing a low pedal. Sometimes, an inexpensive brake flush will solve your problem.

Lastly, the brake hoses. A collapsed brake hose is the hardest thing to diagnose since you can’t see inside of it. The inside may have a tear that allows fluid to travel to the caliper, but will close and not allow fluid to come back (veins and arteries operate this way so that blood goes in one direction, but not the other). If your mechanic uses hose clamps when doing a brake job, make sure it is specially designed for this application. Locking pliers can easily collapse the line. The clamp should have rounded edges to prevent damaging the hose. I think that using hose clamps is a great idea and prevents contaminated fluid from traveling back up the hoses when the pistons are pushed back.

Finally, the cause of most hydraulic problems comes from age and/or brake fluid contamination. Most manufacturers recommend flushing brake fluid every 30,000 miles, at least. Different conditions may dictate it be flushed sooner. Visually, it should look clear and slightly amber. A test strip can tell you how much moisture is in it, but dark or contaminated fluid is usually enough to tell you to replace it.

There is one more component to the brake inspection: bearings and seals. Read on to find out more.

5 responses to “Brakes: Part 3 (Hydraulic inspection)

  1. You should add pictures to future blog entries. But really good read.

  2. My car developed an occasional high pitched whine which would happen when I was applying the brakes– this occurred directly after having a brake job done at the dealer. Although I told them immediately, they could not initially find the problem. It has now (couple months later) been diagnosed as a brake booster.

  3. Looks like you are a true pro. Did you study about the theme? lawl

  4. I think my brakes are glazed, I replaced the brake pads but then did some pretty heavy driving in the days following that- Im going to take off the wheel and have a look- will rubbing the pads down with emery paper do the job?

  5. I have recently replaced my driver-front brakes with a new caliper, brake pads and rotor. After I bled them. I drove it for a day around town and the hubcap started over heating, smoking and smelling bad. I bled the brakes again, and it was fine for about a week before it started doing all that stuff again. What’s wrong with it?

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