Diagnosing a hard brake pedal is an all-too-common phone call we receive. Brake booster and vacuum-related faults is a process that necessitates a methodical approach, involving logical thinking and a return to fundamental principles. The Booster is often too quickly blamed, so let’s break it done and determine what really is at fault.

How to diagnose a hard brake pedal

18/02/2025

Written by Darren Caldwell

Hard Brake Pedal?

Diagnosing brake booster & vacuum related faults requires some logical thinking and going back to basics to get an understanding of the vacuum supply system & how a booster works.

The Brake Booster

In very simple terms amplifies the brake pedal output force by a difference of pressures across a diaphragm. When a booster is in the stationery position it is vacuum suspended which means there is vacuum both sides of the diaphragm. When the pedal is depressed on the front of diaphragm, we have a negative pressure (vacuum) and on the rear of the diaphragm we introduce a positive pressure (atmospheric pressure). Boosters come in all shapes and sizes and can be single or twin diaphragm.

Vacuum supply

In a petrol vehicle is generally derived from the motor but not always. Some petrol vehicles use engine driven vacuum pumps. Diesel vehicles use engine driven vacuum pumps as a source of vacuum.

“A car comes into the workshop with a hard
brake pedal, it must be the booster. Right?”

This catches many technicians out when diagnosing a hard brake pedal. Go back to basics and check the vacuum supply first. This requires a gauge and an understanding of how the vacuum lines are plumbed and the location of the check valve. A minimum of 20” or 67 KPA is required for a booster to perform correctly. On diesel vehicles be particularly mindful of the vacuum pumps recovery time.

Check the Booster

For functionality & leaks the first basic check is exhaust the vacuum from the system and with your foot on the pedal start the vehicle. The pedal should drop as the vacuum builds up. Some vehicles will experience the pedal dropping nearly to the floor, but this is not a faulty booster. A booster can present with 2 types of leaks. A stationery leak means it is leaking constantly in the off position. This means the vacuum pump or source will be constantly trying to overcome this leak and will usually mean an overall lack of braking performance. A leak in the applied position will always mean a hard pedal and a drastic reduction in brake performance. Both faults can be diagnosed with a vacuum gauge and a basic understanding of the vacuum supply system. Smoke machines can be a handy tool for finding leaks in the stationery position.

Here at Partswise we not only offer new and remanufactured automotive brake solutions, we also offer to the trade the very best in technical support. If you’re still stuck diagnosing the issue, please use the Technical Support form and we will get back to you. If the booster ends up being at fault. We offer a large range of Remanufactured Boosters. Use our Parts Lookup to found out if we stock one for your car.