Brake pedal soft after SBC service

[Motor Head]

Good evening Star Tech contributors,

I have been skimming the threads looking for insight on the SBC braking system but could not find anything that addressed my problem so here goes.

Recently took the wife's 2011 SL 550 in for a B service. The car has 45K miles on it and the brakes have always been very good with a nice firm pedal. Although the pads were still serviceable, the fluid was due for a change and the dealer performed the service.

When we got the car back the brake pedal was "soft" if there is such a thing on an SBC car. To be more specific, the pedal travel has increased noticeably before braking action occurs. When I first got back in the car I accidentally overshot the front side of a crosswalk because I was expecting the car to react to the usual amount of brake pressure but nothing happened until I panic stopped.

We took the car back and the dealer was very cooperative and reran the car through all the brake system diagnostic checks which passed and performed multiple test drives. Their conclusion was that nothing is wrong but I am certain that we went from a very short-travel pedal to a very long-travel pedal for the same braking action after the service.

The only data the dealer doesn't have that we do is that they did not drive the car before the service so have no basis for a before/after comparison.

I understand that there is a mechanical fail-safe mode but I'm looking for advice about how to further investigate the issue since we are less confident with the braking system performance. I am hoping there is a way to restore brake pedal feel as part of the system adjustment. I'm not familiar enough with SBC to know what to ask the mechanic so I would greatly appreciate your input.

Thanks,
Motor Head

[duxthe1]

The brake pedal in SBC cars is all but completely divorced from the braking hydraulics. What you feel in the pedal is a simulated pedal feel and not actually feedback from the brakes. Only at the very end of travel is a small portion of very unassisted emergency braking pressure.

A sinking brake pedal is a symptom of a failing SBC module. I've had this exact complaint on several cars and not been able to duplicate it nor had it leave a fault code trail. They've all ended up needing the SBC replaced. I'd think yours should still be under warranty. You should go back in a huff, make a stink, and have them replace it. Maybe even discreetly mark yours to keep them honest.

[Motor Head]

duxthe1,

Thank you for sharing your experience. Since the brake pedal felt good prior to the brake fluid bleeding service, do you think there is a chance they could have introduced air into the system or, in fact, damaged the SBC module? I feel pretty confident that the change occurred during the service.

[duxthe1]

I'm not going to say impossible, but it tends to happen out of the blue. Assuming the tech was competent it's likely coincidence.

[MTUpower]

What years and models have the SBC? Early 2000's W211 (all models?) and what others? Thx!

[duxthe1]

211 through 2006 and 230 SL through early 2011

[Motor Head]

Here is an update for those interested in SBC issues. I did some research and learned that the pedal is directly connected to a master cylinder and reservoir called the Brake Operation Unit (BOU). It is also were the reservoir for the system is as well as the sensor that tells the SBC how the driver is operating the brake pedal.
Two electro-hydraulic switches isolate the master cylinder from applying actual pressure to the brakes unless a system failure is experienced. Then you only get pedal pressure routed to the front disks from tandem pistons in the master cylinder so you can make it safely to a repair facility.
There is as third piston in the master cylinder portion of the BOU and it is their to simulate the feel of a normal brake pedal. During the procedure to bleed and check the SBC system for air, the STARTEK system asks the technician "if the pedal has soft resistance".
Needless to say I took duxthe1's advice and went back to the dealer. We had a very interesting discussion and the car is there now where it is getting a complete SBC diagnostic check. I'll report out again when I get the result from their findings.

[Motor Head]

Here is a picture of what the master cylinder in the BOU looks like.

[Motor Head]

Trying again to attach the picture