Last week I was taking my 986 to the dragon

[papasmurf]

and got about 2 hours away and the brakes suddenly went soft on the highway. I pulled over and got off the highway and drove about another 2 miles where it happened again with the brakes going soft and pedal going mostly to the floor. Each time this seemed to happen for about 1-2 seconds where upon pedal pressure came back. I limped the car around at low speeds but there were several times where the steering would jerk slightly as if the driver side front brake was locking and releasing along with a popping sound as it happened. The sound was apparent when leaving a complete stop too as if the brakes had frozen slightly similar to how your brakes rust over after a car wash. Shortly before this happened while sitting on a slight incline in traffic I got the impression that the brakes were dragging as I figured I should have been rolling based on the angle of the road when the brakes were released or at least rolling more than I was as it seemed the car was slowing more than it should without brake assist. Of course this killed my trip to the dragon and I wound up having to ship the car back to Texas. I got a hotel and jacked up each corner of the car and the fronts were definitely dragging. The rears from what I could tell were fine though at the moment I had the parking brake engaged so it didn't tell me too much. I pulled the ABS fuse just to see if the condition would improve and if the car could be limped home but within miles it was obvious the problem was coming back and I am pretty sure it was getting worse the increased heat of the brakes dragging. The car is now home and I scanned the ECU with my durametric today and got code 0501. I was thinking maybe this code popped up because of the fuse being pulled so I cleared the code and pulled the fuse but it did not trigger a check engine light apart from the ABS/TC lights. I drove the car about a mile just to see if I would get a CEL again. From the research I did, 0501 is the left rear speed/abs sensor? Would this impact the front brakes? FWIW I did just replace the rear axles before the trip but the car drove about 12 hours at least before having any issues? Is there some specific tests I can do with the durametric to get to the root of the probem?

[tcoradeschi]

I would suggest that you start by making sure that the brakes are mechanically correct.

[78F350]

I've had two cars that had a similar brake issue, but also had the brake pedal stick a little bit. The problem was corrosion in the brake booster from water seeping in. Are the drains in your battery area clear?

[papasmurf]

not describe them as clogged and I even poured water through them just to verify. I remember reading a week ago (when I was stranded) about someone taking their brake booster apart to find the inside rusted/corroded. Everything looks brand new in the car (120k miles) but I know the inside may tell a different story. For the past few years this car has been garaged pretty much anytime it is not being driven and the only water it sees is during washing. I have not driven it much over the past year as I had a fuel pump die and a mouse chew through the sensor wire for the fuel filler valve about a year ago. I see no evidence of chewed wires near the battery or booster thankfully. I did jack the car up just a moment ago on the driver side and both front and rear were not dragging though the front brake does seem to have a "slow" spot each time it turns but it is pretty slight compared to what it was last week. I am probably going to at least turn/replace the rotors pads if they are worn too much. Is there any test to do on the booster before just taking it out and taking it apart? It seems rebuilt units are a bit scare at the moment from searching.

[Marc_986]

The pedal going soft and to the floor is not what a failing brake booster should do, it should give you a very hard pedal (and increased pressured required on the pedal to brake effectively).

If the issue is in the brakes / fluid overheating due to dragging brakes you should be able to clearly smell the brakes. Also a quick walk around the car should reveal which corner it is as the heat can be felt easily.

ABS will not give you any issue with pedal pressure, so that shouldn't be your concern.

So I agree with the first reply to your post -> check the mechanical (/hydraulical) condition of your brakes.

[blue62]

Originally Posted by papasmurf

not describe them as clogged and I even poured water through them just to verify. I remember reading a week ago (when I was stranded) about someone taking their brake booster apart to find the inside rusted/corroded. Everything looks brand new in the car (120k miles) but I know the inside may tell a different story. For the past few years this car has been garaged pretty much anytime it is not being driven and the only water it sees is during washing. I have not driven it much over the past year as I had a fuel pump die and a mouse chew through the sensor wire for the fuel filler valve about a year ago. I see no evidence of chewed wires near the battery or booster thankfully. I did jack the car up just a moment ago on the driver side and both front and rear were not dragging though the front brake does seem to have a "slow" spot each time it turns but it is pretty slight compared to what it was last week. I am probably going to at least turn/replace the rotors pads if they are worn too much. Is there any test to do on the booster before just taking it out and taking it apart? It seems rebuilt units are a bit scare at the moment from searching.

Your brake booster and the line or hose going to it need to be able to hold a vacuum.
There is most likely also a one way valve as well.
You should be able to test them in place with something like a Mighty Vac.

[Gilles]

Originally Posted by Marc_986

T -> check the mechanical (/hydraulical) condition of your brakes.

+1.. I would start with the master cylinder, brake hoses and then for sticking caliper pistons

[Smallblock454]

As the rear brakes seem to be fine, i would suspect the brake booster is OK. Because front left and rear right and right front and left rear are connected. I suspect the front brake cylinders in the calipers to be bad.

Also, how old is the brake fluid and what type was used? DOT4 and DOT5 should be changed every two years. DOT5 is not allowed not be mixed with DOT4.

[78F350]

I agree that the brake booster is not the likely culprit at this point. I'll add that when mine was bad, I pulled out the vacuum tube and looked inside with a cheap borescope. With a flashlight you could probably observe some condensation inside.
Removed and opened:


For papasmurf (and future readers) can we come up with consensus on a logical diagnostic sequence for troubleshooting dragging brake/brake fade?