MOT defect file
Brake servo or vacuum system defective
Last updated
Quick answer
The servo that multiplies your effort on the pedal is not doing its job, or the vacuum supply feeding it has failed. The brakes still work, but the pedal goes hard and you have to push far harder for the same stop.
- Typical cost
- £150–£650
- DIY viability
- Garage job
What you might notice
- A pedal that feels hard and high, needing real physical effort to slow the car
- A hiss from behind the pedal as you press it, sometimes with a change in engine idle at the same time
- Only one or two assisted brake applications after the engine is switched off, where a healthy system holds enough vacuum for several
- Braking that feels normal at motorway speed but very heavy in stop-start traffic, when engine vacuum is lowest
What causes it
- A perished vacuum hose or a split rubber elbow between the engine and the servo
- A failed non-return valve, so the servo loses its stored vacuum as soon as you lift off the throttle
- A ruptured servo diaphragm, which on a petrol engine often shows up as a rough idle as well
- A worn vacuum pump on a diesel or a turbocharged petrol, where the engine does not generate enough vacuum by itself
Advisory or failure?
A brake servo that is inoperative or defective is a major defect and a failure, and so is a vacuum system that cannot maintain enough assistance for the required number of brake applications. A minor leak that still leaves adequate assistance is recorded as a minor defect. Insecure mounting of the servo or the vacuum pump is also a defect. There is a second route to failure here, and it is often the one that bites: without assistance the vehicle usually cannot reach the minimum service brake efficiency, which for most cars is 50%, so the efficiency figure fails alongside the servo item — and where efficiency falls to less than half of what is required, the grade becomes dangerous.
Can you fix it yourself?
Garage job
Two parts of the diagnosis are easy enough to do yourself. Pump the pedal several times with the engine off to exhaust the vacuum, hold the pedal down and start the engine: the pedal should sink slightly as assistance arrives. If it does not, and you can see or hear a split hose, replacing that hose or the non-return valve is a genuinely cheap fix and reachable in the engine bay. Beyond that it becomes a garage job. The servo itself sits between the engine bulkhead and the master cylinder, so it comes out from inside the car with the master cylinder off, which means opening the hydraulics and bleeding afterwards. On many cars the pedal box has to be partly dismantled to reach the pushrod clevis pin, and there is very little room to work.
What it costs
The cheap outcome is the common one: a vacuum hose is £10 to £25 and a non-return valve about the same, fitted for £60 to £120 all in, so this should always be checked first. A vacuum pump is £80 to £250 as a part and usually accessible, giving £180 to £400 fitted. The servo unit itself is the expensive end — £150 to £400 for the part, three hours or more of labour because the master cylinder has to come off and the system has to be bled, so £450 to £650 is realistic. What pushes it there is right-hand-drive packaging on cars where the servo is buried behind the inlet manifold or the ABS block.
What happens if you ignore it
Unassisted brakes are not brakes that do not work — they are brakes that need perhaps three or four times the pedal force to achieve the same result. In an emergency stop, most drivers simply cannot push hard enough, and the stopping distance grows accordingly. It is worse for a lighter driver, and worse again in a car where the fault comes and goes with engine vacuum, because the pedal behaves differently from one stop to the next. A leaking servo diaphragm also lets unmetered air into the inlet manifold, which upsets the engine's fuelling and can put a car over its emissions limit as well. And because the failure mode is loss of assistance rather than loss of fluid, nothing on the dashboard warns you.
Common questions
- Is a faulty brake servo an MOT failure?
- Yes. A servo that is inoperative or defective is a major defect, as is a vacuum system that cannot maintain assistance for the required number of brake applications. A minor leak that still leaves adequate assistance is a minor defect. Without assistance a car usually cannot reach the minimum 50% service brake efficiency either, so that item commonly fails alongside.
- Why has my brake pedal suddenly gone hard?
- A hard, high pedal that needs real physical effort points at lost vacuum assistance rather than at the brakes themselves. The usual causes are a perished vacuum hose or split rubber elbow, a failed non-return valve letting the stored vacuum escape, a ruptured servo diaphragm, or a worn vacuum pump on a diesel or turbocharged petrol engine.
- How can I test a brake servo at home?
- Pump the pedal several times with the engine off to exhaust the stored vacuum, then hold the pedal down and start the engine. The pedal should sink slightly as assistance arrives. If it does not move at all, the servo or its vacuum supply has failed. A hiss from behind the pedal as you press it points at a leak.
- Is it safe to drive without brake assistance?
- No. Unassisted brakes still work, but they need perhaps three or four times the pedal force for the same stop, and most drivers cannot push that hard in an emergency. It is worse for a lighter driver, and worse again where the fault comes and goes with engine vacuum, because the pedal behaves differently from one stop to the next.
- How much does it cost to fix a brake servo problem?
- Have the cheap outcome checked first, because it is the common one: a vacuum hose or non-return valve is £10 to £25 as a part and £60 to £120 fitted. A vacuum pump is £180 to £400 fitted. The servo itself is the expensive end, at £150 to £400 for the part plus three hours or more of labour, so £450 to £650 is realistic.
This is general guidance, not a diagnosis. The severity DVSA records depends on the specific fault found, so the same item can appear as an advisory on one test and a failure on another. Ask a qualified tester or mechanic to inspect the vehicle.
Has this been recorded on your car?
Check the full MOT history for any UK registration.