NuBenz

So, I just got my car back after 3 days in the service bay. I took it in for a loud squeaking noise coming from what seemed like the brake pedal. I felt that it may have been related to the hydraulic braking system. Remember, this noise was separate from the really faint electronic buzz when depressing the brake pedal.

Here was the fix: "702 AIR IN BRAKE FLUID SYSTEM W TEST DRIVE & LOCATE NOISE TO SBC SYSTEM///PERFORM SBC SYSTEM FUNCTION TEST//R&R LEFT LOWER DASH PANEL///LOCATE INSULATION BOOT LOOSE AT BACK OF BRAKE PEDAL PUSH ROD///R&R BRAKE PEDAL & INSTALL INSULATION BOOT///TEST DRIVE AFTER REPAIR & NOISE STILL PRESENT///PERFORM AIR CHECK FOR SBC FLUID SYSTEM///LOCATE AIR IN SBC SYSTEM///PERFORM SBC BRAKE FLUID BLEED///TEST DRIVE AFTER FLUID BLEED NOISE GONE///"

So, they found AIR in my hydraulic lines! :p Now, this is interesting because I have always noted that my SBC brakes were exceptionally "grabby". In the beginning, it was really hard for me to brake smoothly. But, I learned how to drive the car with the "all or none" brake phenomenon. My wife, however, has been unable to drive as smooth as myself. She had been having trouble modulating the brake pedal.

After having my lines "de-aired", my brake pedal has a whole new feel to it. It's amazing. The pedal is firmer, and the pressure feel to it is much more progressive. Much of the "all or none" brake phenomenon is gone. Having learned the ways of the old pedal, I can now drive the car effortlessly with the new feel pedal. Even my wife notes the big difference.

Yes, the SBC brakes are sensitive, but now they're much easier to control, especially for my wife. She was having real trouble driving/stopping smoothly.

The SBC and hydraulic systems must be working together during normal braking, even though the hydraulic system is listed as a "back up safety system". Is this correct? The way I see it is that the hydraulic system must be "assisted" by the SBC with respect to caliper pressure. In an emergency stop (i.e. slamming on the pedal), the hydraulics actuate braking but the computer monitoring pedal motions tells the SBC to clamp down the calipers at max force to assist the hydraulics.

The two systems must be interelated since my pedal has a whole new feel to it with no more noise!

NuBenz

I have had a chance to drive the car more now since having the brake lines purged.

Prior to the brake work, I would hear a very faint electronic beep with minimal pressure on the brake pedal. Even barely touching the pedal would create the beep.

However, after the brake work, the electronic beep only comes on when depressing the pedal with much force or more rapidly. "Normal" braking moves and pressures do not usually create the faint beep sound.

My impression is that once again, the beep represents electronic wizardry at work with respect to the SBC. With air in my hydraulic lines, more pedal work created less effect at the calipers thus necessitating SBC to "make up the difference".

Now, minimal pedal motions create more hydraulic pressure and thus greater effect at the calipers since there is no air in the lines, and SBC now has to do less work. Therefore, the electronic beep is much more infrequent.

Geez, is this an auto class? I know someone out there will use this info... All you W211 mechanics... please enlighten me on the interaction between the hydraulics and SBC if I am incorrect on how these two systems are interacting.