1 Product
Your Current Vehicle
Or
Select Your Vehicle
Shop Brake Servo by Brand
Popular Models and Vehicle Options for Brake Servo
Shop Brake Servo by Type
Only subcategories containing verified fitment products are shown.
Assistance sits between the pedal and master cylinder
The driver moves an input rod and control valve. A vacuum booster admits atmospheric pressure to one side of a diaphragm while vacuum remains on the other, adding force to the output rod. Releasing the pedal equalises chambers and returns the mechanism.
If assistance fails, a mechanical path usually remains, but pedal effort can be much higher. That is an emergency characteristic, not permission to continue driving.
Booster technologies
| Type | Energy source | Typical application | Service concern |
|---|---|---|---|
| Vacuum diaphragm | Engine manifold vacuum. | Many petrol vehicles. | Hose, check valve and diaphragm. |
| Vacuum-pump assisted | Mechanical or electric pump. | Diesel, boosted and stop-start vehicles. | Pump output, oil ingress and control. |
| Hydraulic booster | Power-steering or dedicated hydraulic pressure. | Selected heavy/high-demand vehicles. | Accumulator and fluid compatibility. |
| Electric booster | Motor and reduction/control unit. | Hybrid, EV and driver-assistance systems. | High current, coding and calibration. |
| Integrated brake-by-wire | Electronic actuator with hydraulic fallback. | Modern regenerative braking. | Stored pressure and diagnostic bleeding. |
Exact fitment and pushrod geometry
A physically fitting booster can still preload the brakes
| Check | Variation | Mismatch risk |
|---|---|---|
| Drive side | Pedal bracket and shell orientation. | No alignment or unsafe linkage. |
| Master pattern | Stud spacing and register. | Leak, stress or non-fit. |
| Output pushrod | Length, tip and adjustment. | Drag or excessive dead travel. |
| Pedal clevis | Pin size, offset and thread. | Binding or poor free play. |
| Assist ratio/shell | Single/tandem diaphragm and diameter. | Wrong pedal response. |
| Supply port | Valve size, angle and hose. | Vacuum leak. |
| Electronics | Travel/pressure sensors and network. | Faults or uncontrolled strategy. |
Symptoms and diagnostic priorities
| Symptom | Servo possibility | Other checks | Urgency |
|---|---|---|---|
| Hard pedal | No assist or stuck mechanism. | Vacuum/pump/hose/check valve and pedal pivot. | Stop and recover. |
| Hiss on pedal | Control valve or diaphragm leak. | Hose, grommet and cabin seals. | High. |
| Engine idle changes | Vacuum leak when pedal applied. | Intake and hose leaks. | Prompt. |
| Brakes drag | Pushrod too long or servo not returning. | Master, hoses, calipers and pedal switch. | Immediate. |
| Sinking pedal | Not usually assist alone. | Fluid leak, master cylinder and ABS hydraulics. | Immediate. |
| Assist after several seconds | Weak pump or leaking reserve. | Check valve/reservoir and control. | High. |
Vacuum supply tests
Measure engine or pump vacuum with a calibrated gauge at the defined point and conditions. A healthy petrol engine may lose vacuum under load; a one-way valve and reservoir preserve enough reserve for repeated applications.
Inspect reinforced hose for soft collapse, oil saturation, splits and loose ends. Ordinary vacuum tube may flatten under heat and suction.
Non-return valve and reserve
The check valve should flow toward the vacuum source and block reverse flow, according to port markings. Test with an appropriate hand pump rather than mouth suction.
After switching off, assistance should remain for the specified number of applications. Rapid loss points to valve, hose, reservoir or servo leakage.
Vacuum pump diagnosis
Mechanical pumps can leak engine oil or lose output; electric pumps have fuses, relays, pressure sensors and duty-cycle controls. Check commanded operation, current and achieved vacuum.
A continuously running electric pump may indicate leakage and can overheat. Do not bypass its pressure control.
Basic functional check
On a conventional system, switch off, apply the pedal repeatedly until it becomes firm, then hold moderate force while starting. A slight pedal drop indicates assistance developing. No movement needs further supply and servo tests.
This test does not quantify boost or apply unchanged to brake-by-wire. Follow vehicle guidance.
Hydraulic booster systems
A hydraulic booster can share power-steering pump pressure and includes an accumulator for reserve. Wrong fluid, air, pump wear and steering faults affect braking assistance. Stored pressure remains after engine shutdown.
Depressurise by the specified sequence before loosening any line. High-pressure injection injury requires emergency treatment.
Electric and brake-by-wire systems
Electric boosters combine motor, travel sensors, pressure control and network commands. Regenerative braking requests can alter pedal feel. Diagnosis needs scan data, stable power and trained high-voltage awareness.
Do not back-drive actuators, open sealed hydraulic units or apply power directly. Service mode and calibrations are mandatory where specified.
Master-cylinder leakage into the servo
A rear master-cylinder seal can leak brake fluid into a vacuum booster without obvious external dripping. Inspect the interface during removal. Brake fluid damages diaphragm materials and paint.
If fluid entered the booster, replace affected parts according to guidance and find the master-cylinder cause. Do not drain and reuse the booster.
Safe removal preparation
| Stage | Control | Prevents |
|---|---|---|
| Record | Save codes, pedal data and pushrod measurement. | Lost setup evidence. |
| Depressurise | Follow vacuum/hydraulic/electric procedure. | Unexpected force/fluid release. |
| Power down | Battery/high-voltage isolation as required. | Actuator operation and shock. |
| Protect fluid | Cover paint and cap hydraulic openings. | Damage and contamination. |
| Support master | Avoid hanging or bending pipes. | Pipe cracks and air entry. |
| Pedal linkage | Record pin, clips, switches and spacers. | Misassembly and poor free play. |
Master cylinder moved versus disconnected
Some layouts allow the master to move forward without opening pipes; others require removal and bleeding. Never force rigid brake pipes to gain clearance. Inspect for corrosion and stress.
If disconnected, use clean caps and renew specified seals. Keep dirt from ports.
Pedal pins and switches
Renew one-use clips and inspect clevis pin wear. Brake-light and pedal-travel switches may need adjustment or calibration. A misadjusted switch can keep lights on, disable cruise or affect stability systems.
Route wiring away from pedal movement and confirm floor trim cannot obstruct return.
Pushrod measurement
Use the approved depth gauge or setting tool between booster output and master piston. Too little clearance creates long pedal; negative clearance preloads the master and blocks compensation ports as fluid heats.
Adjust only if the design permits, lock correctly and record the final dimension. Do not copy exposed thread count from another unit.
Installation controls
| Connection | Correct practice | Error prevented |
|---|---|---|
| Bulkhead mounts | Seat gasket and torque in sequence. | Water leak and shell distortion. |
| Pedal clevis | Fit correct pin, bush and new retainer. | Linkage separation. |
| Pushrod/master | Verify specified clearance. | Brake drag or excess travel. |
| Vacuum valve | Seat grommet and flow direction. | Air leak/no reserve. |
| Hydraulic lines | Use new seals and exact torque. | High-pressure leak. |
| Electrical plugs | Latch, route and calibrate. | Intermittent control fault. |
Bleeding and calibration
If the base brake circuit opened, bleed with specified fluid and sequence, including diagnostic valve/actuator cycling. Hydraulic boosters can have a separate assist-fluid bleed.
Calibrate pedal travel, pressure or booster sensors with stable voltage and correct vehicle conditions. Do not clear faults before saving evidence.
Commissioning
Check free play, brake-light operation, firm stable pedal and reserve assistance. Hold pressure and inspect every line. Start and confirm smooth assist without hiss or idle disturbance.
Verify all wheels release, then perform controlled low-speed and measured brake-effort tests. Rescan and inspect temperatures afterward.
Common mistakes
Common errors include condemning the servo before testing vacuum, reusing a damaged check valve grommet, misadjusting pushrod length, bending master pipes, losing pedal clips, ignoring fluid inside the shell and skipping electric-booster calibration.
Do not fit a larger booster to change pedal effort without engineered master-cylinder, clearance and brake-balance analysis.
UK MOT and roadworthiness
Brake assistance, pedal operation, leaks, warning systems and brake effort can affect roadworthiness and MOT assessment. A hard pedal or self-applying brake demands immediate repair.
Even if the mechanical fallback can stop the vehicle, required effort may exceed safe control. Arrange recovery rather than experimenting in traffic.
Practical brake-servo FAQs
Q: Does a hard pedal prove servo failure?
A: No. Test vacuum, hose, check valve, pump and control first.
Q: Can the car brake without assistance?
A: A fallback may remain, but effort and stopping distance can increase dangerously.
Q: Why does the engine idle change on braking?
A: A vacuum hose, control valve or diaphragm may leak.
Q: Should the pedal drop when starting?
A: Slight movement is a basic sign on many conventional vacuum systems.
Q: Can any similar-size booster fit?
A: No. Ratio, mounts, pushrods and master geometry must match.
Q: What causes brakes to drag after replacement?
A: Excessive pushrod length or linkage/master faults can hold pressure.
Q: Can pushrod length be guessed?
A: No. Use the specified gauge and dimension.
Q: Does a check valve have a direction?
A: Yes. Install according to its markings and vehicle flow.
Q: Can brake fluid enter a vacuum servo?
A: Yes, through a leaking master-cylinder rear seal.
Q: Must an electric booster be calibrated?
A: Many require service mode and sensor calibration.
Q: Is hydraulic booster fluid brake fluid?
A: Not necessarily; use each system's exact specification.
Q: Can a continuously running vacuum pump be ignored?
A: No. Find leakage or control faults before it overheats.
Q: How is repair verified?
A: Check assist, reserve, free play, release, leaks and measured brake effort.