In Summary
  • My BMW X5 2003 model started having a soft feeling of the brake pedal and failure to brake on time.
  • My mechanic checked all the brake system and the brake pads were okay.
  • He suggested that we replace the master cylinder which was leaking fluid.
  • However, even after replacing it the car is worse and he says he cannot bleed it.

Hey Paul, a few days ago my BMW X5 2003 model started having a soft feeling of the brake pedal and failure to brake on time. My mechanic checked all the brake system and the brake pads were okay. He suggested that we replace the master cylinder which was leaking fluid. However, even after replacing it the car is worse and he says he cannot bleed it. What could have led to this and how can I sort this out? Brian Isiko.

Hello Brian, good brake fluid works with the brake booster and master cylinder to provide hydraulic pressure which engages brakes, lubricates and cools the brake master and slave cylinders as well as prevents corrosion damage of rubber seals on the above cylinder pistons. Delay to renew brake fluid (recommended time every three to four years) risks damage to brake system seals and cylinders from corrosion, overheating and reduced hydraulic pressure (braking performance) due to buildup of moisture and ensuing air pockets in the hydraulic brake fluid.

That is probably why your master cylinder failed. Going forward for BMW, Mercedes, VW and most modern car brands the service or replacement of brake fluid and brake cylinders requires a specialised electronic brake bleeding tool. This specialised equipment uses recommended pressure to replace the aged brake fluid with new fluid without leaving vacuum in the system or damaging the seals of the cylinders.

An attempt to do this manually will have some degree of success but risks damaging the new seals or building vacuum in the brake lines which prevents prompt buildup of brake fluid hydraulic pressure. This is probably the case in your situation. Find a garage in town with the brake service and bleeding tool which looks like the one in the photo below.