The ABS, TCS, and VSES are all tied together in the CTS. While they work together to keep the shiny side up, they also work individually and have their own sensors, wiring and trouble code system to aid in diagnosis. The EBCM is the brains of it all and will throw trouble codes and shut down the ABS system in the event of a failure. Normal braking will not be affected and as the OP has stated, there are no trouble codes present for any vehicle system. The amber ABS light would be on if there were a problem with the ABS; The service stability system message would flash if there was an issue there and the service traction control system message would flash if there was something wrong there. Since none of these warning messages or lights have appeared, before jumping into expensive parts replacement, check the basics for what typically causes brake drag..... Misadjusted brake pedal rod, misadjusted brake booster rod, or a malfunctioning brake booster (a hard brake pedal would result also). If all of these check out, I would start to look at the EBCM or the brake pressure modulator valve as the next possible culprits.
Agreed, been there done that. With respect to the booster - the power brakes work just fine. With respect to the booster rod going into the master - it's never been touched and it's so unlikely that I'm going there (we know the full history of the vehicle - it's a creampuff and none of these areas have been touched).
Just to recap (again) the calipers were checked - the slides are as loose as a goose. The 4 pistons were checked - they are as smooth as silk. The stainless brackets the pads rest on - cleaned, lubed and adjusted.
From a brake mechanical standpoint - it's really, really unlikely that a pair of calipers are going to intermittently drag at the same identical time. Same thing with the brakes hoses - that would mean both front brake hoses are intermittently failing at the same time - it's not going to happen.
The master cylinder was taken out of the equation because I was able to get the car to exhibit the drag problem and was able to quickly crack the lines at the master - the pressure at the calipers was not released.
This means that the pressure is being held lower in the brake system.
What's below the master and before the brake hoses? The brake pressure modulator and the EBCM. The EBCM is not going to typically hold pressure to the front brakes unless the software program that drives it is faulty.
If it were a TCS engagement it would occur at the rear wheels - it's not. That alone takes TCS out of the equation.
TCS does not apply the front brakes - it applies the driven brakes (rear wheels) and only as a last resort.
In theory, the Stability System could activate the fronts as it's designed to send differential pressure to the front brakes. However, the vehicle will exhibit the issue being driven in a straight line at normal speeds with the foot off the brakes and absolutely zero lateral load. Not only is there no reason for either TCS or VSS to be engaged - they're not. There is no information on the dash indicating engagement.Think grandma driving up a long straight hill and you'll have the right picture
EBCM or brake pressure modulator valve - the modulator valve is going to be replaced
The EBCM could either be reflashed or replaced and the BPMV would just be replaced.... BUT.... I would not throw money at them without having it properly diagnosed.... they are expensive parts and it would be a costly mistake if these are not the problems.
There's nothing left to diagnose
The facts are this simple. It's not the master and it's not anything below the front brake hoses. There is only item left between the master and the hoses and that's....the brake pressure modulator.
To make it even more decisive, when I was able to get the vehicle to do it (when I got home) and removed the lines to the master the vehicle was turned off.
None of these systems were powered up. What then can hold pressure to the front brakes when it's not the master and it's not anything from the brake hoses down?
Only the ABS modulator can do that. It's the only thing that can still maintain pressure against the front brakes when the car is physically turned off and not moving and the master has been disconnected.
The master cylinder, being disconnected, removes the master cylinder from the equation and also everything attached to it. This includes the brake booster and the rod going from it to the master.
So, we can now take out the entire master cylinder and brake booster assembly from the equation. We can also take out anything from the front brakes hoses down. We can also take out any modules being active because the car is turned off.
What's the only item left that can maintain pressure at the front brakes under such conditions? The ABS Brake pressure modulator. It's not physically possible for any other item to do this when the master cylinder is disconnected and the car is turned off. The pressure has to be held lower in the hydraulic system and the only thing left is the ABS BPM.
This is not diagnosis by replacement. A great deal of time and thought has gone into this intermittent issue. Along the way there have been several excellent suggestions that, while being incorrect, caused me to understand the system in ways I had not thought of. However, in each case the culprit can really only be one thing at this point - the ABS Brake Pressure Modulator.
One other key point, even though none of the other systems (TCS, VSS or what have you) were active when this issue has arisen, the interesting part is that every one of these systems uses the ABS Brake Pressure Modulator as it's method of activation. It's the common component and without it none of these systems would work. I'm digressing because it really comes back to being this simple;
If we know the front brakes from the hoses down to the calipers are free and not causing the pressure to be held, and we have disconnected the master cylinder lines, and the vehicle is turned off, and the front wheels are still dragging considerably then we know, beyond any reasonable doubt, that it HAS to be the Brake Pressure Modulator that's maintaining the pressure to the front brakes. There is NOTHING else it can be.
That's not diagnosis by parts replacement, that's very precise diagnosis by any measure of any shop with a truly brilliant driveability technician. In fact, I wish I knew of a shop that would do diagnosis like this as I'd rather not have to do it ;-)
To mmiller... if your buddy has a good scan tool that can read ABS and traction control systems and you know how to interpret the info, I would spend some time with it before buying parts. Problems like this are a bear to diagnose when you don't have any trouble codes or apparent system issues to start with.
See above (in bold ;-) but I did take it down and scanned it myself. I was primarily looking at the Steering Wheel Angle Sensor because I had replaced that a few years back and, just for giggles, wanted to check it. It was reading properly.
Unfortunately it was a lower end Snap-On tool and so I could not activate any of the ABS brake functions individually. (I would have loved to activate valves in the BPM manually several times to see when it would keep the pressure to the fronts).
A specific shout-out to Flavoade - my cousin was an expert driveability tech at a GM dealership before I sold him his current shop. I mentioned your specific case to him and that I had felt that one issue could be causing your entire spectrum of problems. He agreed with me right away and picked off a wheel speed sensor as the likely culprit. He told me that they often fail at low speeds (not enough power to generate a signal) and that the vehicle will often interpret this as a traction problem because the wheel drops 'off the chart' causing all sorts of system to activate.
I felt right from the start that the whole gamut of your symptoms could be caused by one item - he strongly felt the same and picked it off right away. From your description it sounds like all these events happen at low speeds and that's exactly where he said vehicle speed sensors typically fail (or drop off). So I'd encourage you to look into that - it might well be that simple and that cheap to address all your issues in one shot.