What Causes Soft Brakes After Changing Brake Fluid?
4 Answers
The main reason for soft brakes after changing brake fluid is: errors during the replacement process caused air to enter the brake lines, which needs to be promptly purged. The method for bleeding brakes: 1. Two people are required to cooperate to avoid mistakes. 2. First, one person presses the brake pedal 4-5 times, while the other loosens the bleeder screw on the brake caliper. 3. If there is indeed air in the brake lines, some brake fluid and gas will spray out at this point. 4. Repeat the above steps. When only brake fluid sprays out, it indicates that all the air in the brake lines has been expelled. Finally, tighten the bleeder screw. 5. After this, the soft brake issue will be noticeably improved.
Last month after changing the brake fluid, I also encountered this situation - the pedal feel became spongy which was quite scary. The mechanic told me the most common cause is incomplete air bleeding, where tiny air bubbles remaining in the system will lengthen the brake pedal travel. Another possibility is using different specifications of brake fluid or mixing brands, which can alter the boiling point and affect pedal feel. It could also be that during the fluid change, aged hose connections were loosened, causing minor leaks. He advised me to return to the shop immediately for vacuum-assisted re-bleeding, especially emphasizing never to use the pedal-pumping bleeding method as it doesn't fully remove air. After redoing the procedure, it indeed returned to normal.
As someone who frequently works on brake systems, I've found that a spongy pedal after fluid changes is 80% likely due to improper operation. Air pockets are the main culprit, especially stubborn bubbles trapped in the ABS module. Another possibility is mixing incompatible DOT-rated fluids - like combining DOT5.1 with DOT4 can create gel-like blockages in the lines. Sometimes incomplete flushing leaves old fluid mixing with new, creating vapor lock. Last time I encountered this, I had to loosen all four caliper bleeder screws while my assistant used professional pressure-bleeding equipment - took a solid 30 minutes to purge those stubborn bubbles. My advice: address any abnormalities immediately, don't delay.
In my ten years of car enthusiasm, I've encountered brake softening after fluid changes twice. The first time was at a roadside stall where the master cylinder seal was later found corroded and leaking. Learning from this, during my second DIY attempt, I strictly used OEM-specified brake fluid, bled starting from the wheel farthest from the driver's seat (each caliper twice), and maintained reservoir levels religiously - never letting lines run dry. Final pedal check must be rock-hard. If spongy post-change, never drive - dangerously extended stopping distances will occur.
