Replacing a master cylinder and servo unit (2024)

Dual-circuit cylinder and servo

If a master cylinder is leaking replace it. The cylinder is usually mounted on the bulkhead separating the engine and car interior. It may be fitted with a vacuum servo unit.

The master cylinder is normally connected to the brake pedal by a pushrod. On some cars, particularly those originally designed for left-hand drive, it may be positioned on the nearside of the engine compartment and linked to the pedal by a cross rod.

A leaking or faulty servo unit should also be replaced. It may be faulty if the brake pedal is hard to push down, and all other brake faults have been eliminated (See Bleeding the brakes).

One-circuit cylinder

Before replacing it, check the condition of the air filter, which may be causing sluggish operation. It should be changed every three years, 36,000 miles or 60,000 km.

Before you disconnect brake lines or electrical leads, such as those to the stop-lamp switch or fluid-level warning light, make a sketch of where each fits. If the car has twin brake lines, this is vital. Tag them, and mark the cylinder body accordingly.

Be careful to note the positions of washers, bushes and pedal return springs. Check how the pushrod is fitted to the brake pedal - there may be more than one hole in the pedal arm.

If you empty the brake fluid from the master cylinder, do not use the fluid again.

Preparing to remove the master cylinder

The brake fluid can either be drained from the reservoir or left in and sealed. Generally it is better to drain the complete system.

To drain the cylinder, open the offside front-wheel bleed nipple. If the car has a split system, open the offside rear-wheel nipple as well. If in doubt, open each bleed nipple in tum, front first then rear.

Put a jar under each nipple and pump the brake pedal until the cylinder is empty. Avoid spilling brake fluid on the car paintwork, for it is corrosive. If some is accidentally spilled, wipe it off at once.

If you leave the master cylinder full, seal the breather hole in the breather cap with adhesive tape. Or screw the cap down over a thin sheet of plastic.

A small amount of fluid will still escape, so put plenty of rags under the cylinder and the brake lines.

On some cars the reservoir is a separate unit that can be unplugged from the cylinder and refitted to the replacement.

If it is full, seal the breather hole, and as you remove the reservoir quickly seal off the bottom outlets with your fingers, to catch drips.

Disconnecting a pushrod

On most cars, the pushrod is linked to the brake-pedal arm by a clevis pin that is held in place by a split pin or clip.

The linkage may be so high up the pedal arm that you have to remove a parcel shelf or trim panel to reach it.

Straighten the split pin and pull it out with pliers. Push the clevis pin out sideways.

When reassembling, use a new split pin, and be sure to link the rod to the correct hole.

If the linkage is hard to reach, try pushing the pin through a strip of adhesive tape, then wrap the tape round your finger to hold it while you locate the hole.

Disconnecting a VW pushrod

On some VW cars, the pushrod can be pulled straight out of the master cylinder; it does not have to be detached from the brake pedal. Check the type in the workshop manual.

When reassembling, you may have to adjust the length of the pushrod. Slacken the locknut and turn the rod so that there is about 1/25 in. (1 mm) of free play between the ball end of the rod and its seating in the cylinder piston.

On cars with a tandem master cylinder, make sure the pedal-stop adjustment allows the pedal full movement, so if one system fails it can operate on the other.

Removing the master cylinder

Uncouple the brake pipes to the master cylinder; seal the pipes to prevent spillage, or allow the fluid to drain into a jar.

Disconnect any electrical leads, such as those to the fluid-level warning light or stop-lamp switch.

You may also have to disconnect any other parts that are in the way, such as the choke or throttle cables.

Generally, the master cylinder is secured by two nuts and studs, or nuts and bolts. Sometimes there is also a bracket holding it to the inside of the wing.

On VW Beetles, the master cylinder is inside the front luggage compartment and is unbolted from inside the front wheel arch. Remove the wheel to get at the nuts.

Remove the bolts holding the cylinder and lift it off, taking care not to drop any washers.

After fitting the new master cylinder, screw up pipe unions by hand at first, then tighten them with a spanner they are easily cross-threaded.

When reassembly is completed, refill an empty reservoir with brake fluid. Whether or not the reservoir has been emptied, bleed the brakes. Top up the reservoir.

Replacing a servo unit

When renewing the servo unit alone, it may be possible to dismount the master cylinder without disconnecting the brake pipes - they may be long enough to allow it to be moved to one side.

If so, there is no need to drain the master cylinder. If not, disconnect the push rod and remove the master cylinder as described.

If there is a seal between the servo and the master cylinder, check its condition and renew if necessary.

Make a sketch of the way the servo pushrod is connected to the brake pedal, and how the vacuum hose is connected to the servo. Disconnect them both.

Unbolt the servo unit from its bracket on the bulkhead and withdraw it. If there is a gasket between the servo and the bulkhead, check whether it is worn; renew if necessary.

When refitting a new unit, inspect the condition of the vacuum hose and renew if necessary. Make sure the clips are tight.

If the master cylinder has been disconnected, bleed the system after reassembly is completed.

Disconnect the servo unit from the master cylinder. Check the condition of any seal between the servo and cylinder, and fit a new seal if necessary.

Note how the vacuum hose is connected to the servo, and then disconnect it by slackening the hose clip.

Note how the servo unit is linked to the brake pedal, and then disconnect it. Unbolt the servo unit from its mounting braket. Withdraw the servo unit. If there is a gasket between the servo and its mounting, check condition and renew if worn.

Renewing a servo air filter

On most later cars, there is a felt air filter fitted on the rear of the servo unit, encircling the pushrod.

It is not usually necessary to take off the servo in order to renew the filter. You should be able to reach it either between the rear of the unit and the bulkhead, or from inside the car.

The filter is covered by a rubber gaiter. Slide the gaiter along the pushrod to expose the filter, and prise the filter off.

Use a sharp knife to slit the new filter across its radius, making the cut at an angle of approximately 45 degrees.

Slip the filter over the pushrod, position it in its housing, and press the two ends of the filter together. Replace the rubber gaiter over the filter.

Replacing a master cylinder and servo unit (15)

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Replacing a master cylinder and servo unit (2024)


Is it difficult to change a master cylinder? ›

Brake master cylinders provide the power behind your vehicle's braking system, so when they go bad, it can be a serious issue. But luckily, replacing them yourself isn't overly complex and should be more than possible if you have basic automotive knowledge.

How long does it take a mechanic to replace a master cylinder? ›

The master cylinder is essentially the lifeline of your brake line, so if it breaks down, it will take far longer to fix than most other repairs. Depending on the vehicle and the extent of the replacements needed, you can expect somewhere between 2 hours or an all-day repair.

Is a brake servo the same as a master cylinder? ›

No, a brake booster is not the same as a master cylinder. A brake booster helps increase the pressure in the braking system, making it easier for the driver to stop the vehicle. A master cylinder contains fluid that is used to apply pressure to the brakes.

What to do after replacing a master cylinder? ›

Brakes must be bled after master cylinder replacement. Purchase a bleeder kit from an automotive supply store. Follow the directions that come with the kit.

What is the average cost to replace a master cylinder? ›

The average cost for brake master cylinder replacement is $841 to $944. Enter your vehicle's information to see how much brake master cylinder replacement costs in your local area.

What are the symptoms of a bad master cylinder? ›

Inconsistent Brakes - When a master cylinder begins to fail, sometimes the brakes will feel fine one second and lose braking power the next. If the fluid is leaking past the seals inside the cylinder, the pedal may feel firm for a moment but won't hold steady; it'll feel spongy and keep sinking towards the floor.

Can I drive my car with a bad master cylinder? ›

This cylinder is the principal valve through which brake fluid is pushed to make the calipers press brake pads on the rotors. In essence, this means it plays a significant role in making the car stop when you apply brakes. Any problem with the master cylinder can lead to an accident while you are on the road.

What is the life expectancy of a master cylinder? ›

Master cylinders should last between 70,000 and 110,000 miles. They could last forever, but they have rubber seals that wear out over time. A bad master cylinder compromises your vehicle's stopping power.

Where is the brake servo located? ›

The brake servo is a unit placed between the brake pedal and the brakes. It is the flattish, round, frying pan shaped thing which sits under the bonnet, against the bulkhead, and is attached to the brake fluid reservoir in the bonnet and the brake pedal inside the co*ckpit.

What does a servo do in a car? ›

In modern cars, servo motors are used to control its speed. When stepping on the gas pedal, it sends electrical signals to the car's computer. The computer then processes that information and sends a signal to the servo attached to the throttle to adjust the engine speed.

What is a brake servo unit? ›

The brake servo reduces the force you need to apply to the pedal when braking. The negative pressure in the engine's intake manifold is used to achieve this. Both pneumatic and hydraulic systems exist. In cars with power steering, the servo pump may be used for this purpose.

Do I need to bleed brakes after replacing a master cylinder? ›

Assuming a complete brake conversion is being completed, the first thing to do is bench bleed the master cylinder. Once that has been completed and everything installed, then the bleeding of the rest of the system can be done. Bench bleeding the master cylinder is the first thing that needs to be done.

Why does my brake pedal still go to the floor with a replaced master cylinder? ›

If, after the master cylinder has been replaced and the system has been bled, and still your brake pedal goes to the floor, the new master cylinder may be defective. This is rare, but it happens and the new cylinder should be examined.

Why is my brake pedal soft after changing master cylinder? ›

The most common reason for a soft brake pedal is simply air still in the system. The easiest way to diagnose this problem is to pump the brake pedal gently a few times. In doing so, the pedal should become firmer with each gentle press of the pedal. If it does, then the necessary answer is bleeding the brakes.

Do you have to bleed brakes when changing master cylinder? ›

Make sure the master cylinder is bench bled prior to installation. Follow the vehicle's manufacturer-specified bleeding procedure. Some automotive manufacturers require a separate ABS bleeding procedure. Failure to follow manufacturer procedures may cause a “no pedal” condition after a new master cylinder installation.

Can I drive a car with a bad master cylinder? ›

This cylinder is the principal valve through which brake fluid is pushed to make the calipers press brake pads on the rotors. In essence, this means it plays a significant role in making the car stop when you apply brakes. Any problem with the master cylinder can lead to an accident while you are on the road.

Is replacing a clutch master cylinder hard? ›

If your vehicle's clutch master cylinder fails, you should conduct a clutch master cylinder replacement or repair. Luckily, this isn't as hard as it sounds, but you should do it as soon as possible to keep your vehicle driving safely and smoothly.

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