Initially I responded to a year-old post because it was very similar to what my car is doing. However, OP of that thread said only the front brakes were being affected. I think all four of my brakes are affected.

I'm trying to determine if this is:
1. The brake booster
2. The master cylinder
3. ABS unit (the brake dragging coincides with ABS light recently becoming unlit (the light had been coming on intermittently, less and less over time, then gone completely after having car up on stands for other work for about three weeks)

Here's what it's done today:

Morning drive: Brakes started dragging after about 30 minutes of mixed driving. Not dragging much...but enough that the car would not roll if put in drive or reverse with foot off the brake. Was noticed when driving in a 25 mph zone and noted the car was slowing as if light pressure was on the brake pedal - on it's own while coasting.

So I did some tests:

1. Engine stopped. Pressed brake pedal several times to release vac. Pedal climbed and travel reduced to nothing after several pumps. Pressed pedal hard for more than 20 seconds and the pedal did not move. This appears to indicate the master cylinder is not bypassing fluid internally and that there are no leaks in the braking system (the car completely dry at all corners).

2. Started engine while pressing brake pedal. The pedal sank slightly - normal behavior. Booster and check valve operating normally.

3. Pressed the brake pedal lightly with the engine running. I didn't measure how much the pedal fell over the next 2-3 minutes, but it did feel like it fell a little - but not sure if greater than 10 mm (3/8ths inches). Not enough drop to clearly indicate a problem with the MC.

4. Continuing from #3 I turned off the engine while pressing the brake pedal for at least 30 seconds. The pedal was rock steady, did not fall or rise. Indicates brake booster operating normally.

5. With engine still off I pressed the brake pedal repeatedly and with each pump the pedal rose higher and travel reduced until there was none. Held it for a minute. Normal.

Checked the fluid level in the reservoir. It's just below max and filled with recent ATE racing blue fluid.

Hoses less than 2 years old - they are stainless hoses with a clear outer casing.

Front rotors are drilled Zimmerman - less than 2 years old. Rear are likely original, with slight wear lip.

Afternoon Drive:
Drove it around rural roads near my home (west of Leesburg, VA), not wanting to have brakes lock up on a limited access highway (Rt. 7). But the brakes behaved normally. So for the return home I got on 7 for about 2 miles and got up to 60 mph. Took the off ramp headed home. No issues. But about a mile down the rural road I came to a stop sign and pressed the brake pedal and noticed after initial normal pedal resistance there was a light thunk sound and the pedal simultaneously gave a little...maybe a half to whole centimeter (less than half an inch). So I continued driving but pumped the brake pedal to see if it's feel had changed. I was a bit softer with a bit of a "woof" sound before hitting resistance. But this too went away after driving just a bit and the pedal firmed up and the pedal rose and travel decreased (like the at rest with engine off test - but this time I was driving the car at about 20 mp around my neighborhood)...so the brakes became more touchy...instantly grabbing because of the reduced travel. And the more I pressed and tested the feel of the brakes going round and round I noticed the brakes starting to drag again...while coasting and while going down the gentle hills....the car was holding itself back, when normally it would accelerate gradually.

I hopped out and quickly shot the rotors with the IR gun:
Both front rotors were about 270 +/- 2 degrees F
Both rear rotors were about 130 +/- 2 degrees F

I'm thinking this is probably a MC in the early stages of failing, but wonder if the "thunk" and slight pedal drop at the end of my drive point to a booster in early stages of failure and keeping slight pressure on the system when these thunks and pedal drop happen.

Or...is it something to do with he ABS system miraculously fixing itself since September? The ABS light used to come on after a few minutes of idling or within the first 30 meters of driving. The light would always come on when the left rear wheel hit a bump (so I was thinking loose sensor contact). Throughout winter the ABS light would come on less frequently. By spring it might come on and then go out...including the warning light. Then about 5 weeks ago I put the car up on stands to work on the Bowden cable leak and assess other project areas for this summer. The car has been on the ground for 2 weeks and driven 2-3 times a week. From the first drive after lowering the car back onto its wheels the ABS light has not come back on. This should mean the ABS system is engaged all the time now. I can't help but wonder if some weakened link in that system has been exposed now that system is ON.

I don't want to throw money and parts at the problem. I'd really appreciate some help from those more knowledgable and experienced with these brake systems. I hope to achieve a definite diagnosis and post it here before buying parts. But as these things go...it could be a few things get replaced before the culprit is revealed.

Thanks!

 

Ain't no expert but sounds to me like an ABS problem.

Try removing the a ABS relay in the CE panel and drive the car.
If everything's good, you'll know what it is, if not, well...

 

Bertand -
Thanks for this suggestion. I've done some searching. A VW TDI owner had the same problem and pulled the ABS fuse and the problem went away...but thread ended before solution found. And Caddy CTS owner has the same problem but affecting only the front (which now I think is the case with my car). The CTS owner is questioning about a valve in the ABS that might be sticking intermittently, and only applying slight pressure to the front.

Tomorrow I'll pull the relay and see what happens.

 

Last night I Googled "ABS brakes dragging" and learned a lot. I'm 95% sure my booster is fine. It's possible the MC is failing, but it seems fine, too. But the coincidence of brake dragging when my ABS started working again (as Bertrand suggests) point to an ABS problem. Turns out a wide variety of vehicles experience the same symptoms and it's the ABS "pump" keeping some pressure on the front or all four brakes. But it's often NOT a fault in the "pump"...it's lack of proper bleeding and/or neglecting to change the fluid...gets rusty and sediment fouls the valves in the ABS unit.

Before following Bertrand's advice to remove the ABS relay, I thought I'd try what was suggested on a number of other car and truck forums to correct this - "cycling" the ABS "pump" (this thing is never called just one name in the forums). This is the only way to make sure clean fluid is flushed through all the valves that deliver the pulses to the wheels that the wheel sensors say are locked up.

In the case of my car, the ABS system has not been operational for at least a year. Yet the ATE Super Blue fluid was new when I bought the car. Many home mechanics know how to bleed brakes, but most don't bleed the ABS "pump". Why? Because it requires a special expensive electronic device that gets hooked up to the pump and cycles it to flush it with new fluid (AFAIK, if ABS is not activated during driving situations, the fluid just sits in it indefinitely, and is not flushed by normal brake system flushing).

But there is a method to cycle the ABS pump without the special device - locking up the wheels on an ice or snow covered parking lot or a gravel road. Fortunately, there are miles of gravel road near my home. I cycled the ABS pump at least a dozen times - and now my car is very dusty

I continued to drive it for longer than described in my first post. I tried fast and slow driving, hard braking bursts and light pressure for longer periods. I couldn't get the brakes to drag again. And the IR gun showed cool rotor temps all around.

Today may have been one of those days when the real problem didn't want to play. But given that the brake dragging coincided with the return of a functioning ABS system, I have reason to think this is ABS related. (Note: my earlier tests of the booster were all normal. The MC might be just starting to fail...but not consistently, and it passed some basic tests described at the top).

I will drive it a lot this week and give an update.

 

Jon,

Great diagnostic work on the ABS. You mentioned ATE Blue brake fluid and that it may have been 'stored' in the ABS unit without any flushing for a while. This brake fluid is really much more hygroscopic than many others, so needs to be changed more frequently. I used it for my track car, because of it's higher DRY boiling temp, but changed it in the spring, and again mid-summer (May-November track usage) each year.

I use Castrol LM (I think that's the designation, forgot the exact name. Large white plastic l liter container with red and green markings!!) in my street cars (including PSD), and change this every three years maximum.

 

Gary -
Thanks for responding with the bit about the greater hygroscopic behavior of the ATE Blue - I vaguely recalled something about this fluid was detrimental over time but couldn't remember. Time to fill the PowerBleeder with Castrol LM.

Do you happen to know if the ATE amber fluid (one notch below the Blue on the price and performance scale) is also highly hygroscopic? Asking because I have an unopened bottle (I got a tip from Fred's TDI forum to alternate between Blue and Amber...as the color change tells you when you've finished bleeding each corner ).

 

Jon,

Don't know about the difference - if any - between the blue and gold ATE. I guess I'd be surprised if there was much difference, as I think the DRY boiling points are pretty close together. Only high boiling point fluid I'm aware of is the BMW (made by Castrol, I think) which is about the same price as 'solid' gold bars per ounce!!

I have not been able to find the Castrol LM anywhere locally for several years. Bought about 4 liter bottles a couple of years ago on the internet.

 

Thought I might wait longer to post the update, but I've been driving the car a lot and testing the brakes at all speeds, light pressure, gradually increasing pressure, sudden stops, and trying to do the 0-60 and back to zero in 9 seconds. The brakes are performing perfectly. The problem appears to have been the ABS pump/valves needed to be cycled to purge stagnant fluid and replace with fresh. I don't have the special device that cycles the ABS pump...so I'll cycle mine at least a few times a year on a gravel road.

 

Was the brake fluid reservoir very very full?

 

Reservoir was just below full.

 

The full mark or just below the cap? If overfilled it can drag the brakes when hot.

 

Just below the full mark. Glad you added the info about the affect of overfilling. There are more than a few things that can cause the brakes to drag. So far the circumstances of my ABS system point to the pump needing to b cycled (and owners of other makes have had this problem).

There are several causes of single calipers dragging, but when the front pair drag similarly (indicated by same temperature - shot with IR gun) but not the rear - then it points to the ABS, specifically the pump/valves. Of course it's important to first test the master cylinder and the booster.

I've only had one other dragging brake problem - on a 1966 Mercedes-Benz Unimog. That was a case of old and internally swollen brake lines. Brakes came on, then stayed on and slowly released as the fluid very slowly returned through the constricted hoses. Typically this problem also mostly happens on the front - so it pays to check all the usual suspects.

 

Thanks very much for all your efforts Captain_Slow.

My S4 is off the road for this very problem. I could count the times I've activated the ABS on the fingers of one hand. OK well maybe two hands. That includes the GT.

Now where did I put that gravel road map ... ):-]

.

 

Today my ABS light came on and off intermittently on the way to and from Fraggle's to test my cooling fans final stage. About halfway home, pulling away from a stoplight, I sensed the brakes were dragging again. And the sensations in the brake pedal were the same as the last time this happened - and associated with the ABS light coming on. The light seems to be triggered by a worn wire or poor sensor connection. In any case, the ABS going offline and back on intermittently seems to be related. When the light comes on I feel and hear a light "thunk" and the pedal gives simultaneously. Further applications of the pedal cause it to firm up (lose travel). This is when the dragging starts. Today I lost a good bit of brake pad based on the dust. Temps got up to 300 F on the rotors. I stopped to eat lunch and let them cool off. No problem the rest of the way home.

I'm going to pull the ABS fuse for the remainder of the summer and observe. I want to be sure this isn't the MC or booster. Still seems like ABS related, but time to really make sure.

The ABS light never came on during the entire period I thought I'd solved this by cycling the ABS pump on the dirt road.

Cycling the ABS pump/valves is still something to do at least when fluid is changed (every 1 or 2 years).

Unfortunately, between this and my fan problem (thread: S4 Fan Fun) today has sent me back to square one for both. Bummer...

 

You might try a different ABS relay.