Car Brakes Very Sensitive – 9 Common reasons


Coffee all over the dashboard? Yes, I know, I’ve been that soldier. Brakes so sensitive make you look like a permit driver. Sharp brakes need immediate attention, buckle up, you’re in the right place.

The 9 most common reasons car brakes are very sensitive include:

  1. Overfull brake fluid
  2. Contaminated pads & rotor
  3. Disintegrated pads
  4. Faulty calipers
  5. Faulty brake lines
  6. Blocked master cylinder
  7. Faulty brake booster
  8. Brake pedal rod out of adjustment
  9. Faulty Brake controller

In this post, you’ll learn about the most common reason for brake sensitivity, how to diagnose them and what you can do to fix them.

Overfull Brake Fluid Reservoir

Brake fluid

As brake fluid heats, it expands slightly in volume. When the brake fluid reservoir is filled to the Max and as the reservoir cap is fitted tightly, the fluid has only one place to go. It pushes the caliper pistons out, ever so slightly, just enough to take up the free-play.

 Now applying even light braking force sticks you to the windshield.

Too much brake fluid is so common, as brake pads wear the caliper pistons move further out, this of course requires more fluid to fill the void, and so the fluid reservoir drops a little below Max. Seeing the low level, maybe the owner tops up the fluid, maybe a little too full.

Now consider the vehicle has new pads fitted, the pistons are pushed back into the calipers which cause brake fluid to rush back to the reservoir.

If the excess fluid isn’t removed from the reservoir, the brakes may start to drag or become excessively sensitive.

Diagnose: Check the fluid reservoir, if overfull, drain some fluid using a siphon, careful not to contaminate the fluid.

Contaminated Pads & Rotor

Rusty axle

Pad or rotor contamination may cause braking to become sensitive. Fluids like brake fluid, coolant, diesel, gas, power steering, transmission, gear oil, etc could cause the pad materiel to break down. Similarly, rotor hot spots, damaged, pitted or rough metal could cause the brakes to bite aggressively.

Diagnose: Remove the brake pads and check for oil contamination, check the rotors for damage, raised or rough surface, cracks, warping, hot spots, etc.

Rotors can be refaced if within spec or replaced together with pads.

Disintegrated Pads

Damaged pads

A faulty brake pad is a common problem in old cars that don’t drive a ton. Corrosion causes the backing plate to separate from the braking material. The material can bind in the rotor causing braking issues such as grabbing and sensitive braking.

Diagnose: Remove the calipers and pads and check for separation, replace if necessary.

Faulty Calipers

Brake parts

Calipers do all the heavy lifting when it comes to braking. Faulty calipers are common too. Corrosion build-up on the caliper piston can cause the piston to bind, locking the pad tight against or close to the rotor.

Touching the brakes can magnify braking force.

Diagnose: Attempt to push the pistons back into the caliper using a screwdriver, if the piston fails to retract, if maybe partially seized. But to confirm this we’ll need to check the brake hose. (see below).

Faulty Brake Lines

Brake hose clamp

The brake hoses are flexible, they transmit fluid pressure from the master cylinder to the dynamic suspension and caliper. Cracked and perished hoses, corroded metal fittings are normal with age. The hoses break down on the inside too.

The rubber comes away in strips and often acts like a one-way valve allowing fluid pressure to the caliper but not allowing all the pressure to return. The result is a partially applied brake, which makes the brakes super sensitive to further application.

Diagnose: As per the caliper test, attempt to push back the caliper piston, if it won’t move open the bleed nipple.

If the piston now retracts, the brake hose is faulty and all four brakes hoses will need replacing.

If the piston still won’t retract, the caliper is faulty, best to replace calipers in pairs, on the same axle.

Blocked Master Cylinder

The Master cylinder employs two separate pistons and seals to apply brake pressure to the four calipers. The two separate brake lines are usually paired diagonally.

Hydraulic brakes
  • Port 1 – Front left and rear right
  • Port 2 – Front right and rear left

Master cylinders are a major brake component responsible for the safety of the occupants and other road users. They are very well engineered and don’t generally cause problems.

That said, compensating ports inside the master cylinder may become blocked preventing the brakes from releasing fully. Disintegrating components and contaminated fluid can cause the small ports to block.

Diagnose: Remove the reservoir cap and have a helper repeatedly pump the brakes. Fluid should be seen splashing inside the reservoir as the fluid rushes back into the reservoir through the compensating ports.

Faulty Brake Servo/Booster

Brake booster

The Servo is fitted between the brake pedal and master cylinder. It employs an engine vacuum to reduce foot pedal effort. The Servo uses two chambers separated by a rubber diaphragm. As the engine vacuum lowers the pressure on one side of the servo, the drivers only need to apply light pressure to apply brakes. Internal spring and an atmospheric valve return the pedal to the rest position.

Diagnose: Press the brake pedal several times, now start the engine. The pedal should drop slightly. Release the pedal and check it returns to the rest position immediately.

A Brake Booster (mostly fitted to larger trucks) operates in a very similar way. However, it also employs hydraulic pressure to the master cylinder using a stand-alone pump or scavenging hydraulic pressure from the power steering system.

Brake Pedal Rod Adjustment

Holding down brake pedal

The brake rod is a mechanical coupling between the foot brake and the master cylinder. The rod is adjustable on some vehicles and a specified free-play is required. Adjusting, modifying, or swapping out rods, pedal boxes, master cylinders, boosters, or modifying the thickness of the firewall will affect the brake pedal to master cylinder free-play.

Diagnose: Check if your vehicle is fitted with an adjustable brake pedal rod. Check for free play.

Faulty Brake Controller

Scan tool

Higher-end cars will use a form of active braking. The sensors measure the speed of the brake application to identify an emergency. Another sensor measures brake pedal pressure and determines if the pedal is pressed hard enough. If it calculates that it hasn’t, the controller uses a reservoir of energy to apply greater force to the brakes.

Diagnose: Brake controllers have their own dedicated control module. Use a code reader to check for fault codes. Check out the code reader I recommend here on the “Brake repair tools page” and if you need brake system parts, check out the Amazon link below.

Amazon Brake System Parts

Related Questions

How can I make car brakes stronger? To increase car brake performance, change the brake fluid and flush the hydraulic system. Check and make repairs to the following components:

  • Brake pads
  • Rotors
  • Calipers
  • Flexi-hoses
  • Brake lines
  • Master cylinder
  • Servo

John Cunningham

John Cunningham is an Automotive Technician and writer on Rustyautos.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of classic car ownership, from tires to roof aerials and everything in between.

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