My faithful Accord has developed an intermittently spongy or mushy brake pedal. When driving at modest speeds in traffic, say 10 to 30 mph, the brake pedal will occasionally feel as if there is a leak in the system and slowly depress to the floor. Once this happens and the car approaches 40 mph+, the brake pedal tends to stiffen up just like new and stops well. The car currently has 218k miles, had new brakes and calipers installed three years ago at 185k, and a new clutch slave cylinder at about 150k miles. I have not detected any leakage of brake fluid.
Any ideas out there what this could mean? Could it be possible that the power boost to the brakes is going out? How could this be checked?
I would put my money on a replacement brake master cylinder. The “leak” would be internally — from one part of the brake master cylinder to another. Replace the brake master cylinder and continue to stop when you hit the brakes.