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Soft Brake Pedal

6K views 8 replies 5 participants last post by  JayMac 
#1 ·
Hello. I wanted to do some road-course lapping in my '99 323is on Saturday so to prepare on Friday I bled the brakes and put in about 1L of fresh Motul RBF600. I went to the track and did a few short sessions with the car (mostly drove my Mustang).

My buddy drove the car back and mentioned the brake pedal was soft - specifically "the first inch of travel seems to do nothing". I took it for a spin and sure enough it was soft so I assumed I must've boiled the fluid and introduced air bubbles or not tightened a bleeder tight enough. This afternoon I re-bled the brake system and some tiny air bubbles came out of the right rear (but they could've been from my dad depressing the brake too quickly when bleeding).

Unfortunately this didn't solve the problem and the pedal is still soft. Has anyone experienced something like this after a track day? Initial thoughts are somehow there's still air in the system, or I have a master cylinder issue.
 
#3 ·
Odd. First thing I would suspect would be the process by which you bled the brakes. If you did actually bleed the lines correctly (RR, RF, LR, LF) and still have air getting in, there must be a breech in the system somewhere. A few short sessions shouldn't boil brand new Motul RBF600, so I don't think boiling is the problem here.
 
#4 ·
Dropped the car off at a shop today to check it out as I don't have time until the weekend to work on it and its my daily driver. They re-bled the system (which didn't help really) and checked it over. The tech showed me the wear on the front brakes was quite excessive. The inside pad is much more worn than the outside so I thought I had more pad left then I actually do. He felt new brakes would bring the pedal back and that the master cylinder is fine. I am inclined to agree so I ordered new brakes from Turner today and will report back on the issue once installed. At least I didn't bleed the brakes wrong... that would've made me feel like an idiot since I've done it a ton of times on my Mustang track/weekend car.
 
#6 ·
a quick easy way to confirm the condition of the master cylinder is to pinch off the hoses with appropriate clamps (actual clamp devices or clamping pliers, and, in a pinch, needle nose vice grips with rubber tubing over the teeth to precent damage to the hose), and then depress the pedal.

it should be rock hard. any movement in the pedal indicates the pressure is bleeding past the seals in the master cylinder.

and i agree that uneven pad wear on our vehicles is an indication of caliper slide binding and/or caliper piston sticking.



df
 
#7 ·
a quick easy way to confirm the condition of the master cylinder is to pinch off the hoses with appropriate clamps (actual clamp devices or clamping pliers, and, in a pinch, needle nose vice grips with rubber tubing over the teeth to precent damage to the hose), and then depress the pedal.

it should be rock hard. any movement in the pedal indicates the pressure is bleeding past the seals in the master cylinder.

and i agree that uneven pad wear on our vehicles is an indication of caliper slide binding and/or caliper piston sticking.

df
I thought it was pump the brakes with the car totally off and if the pedal gets hard and stays hard then the master is fine.
 
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