Kind of stumped: one brake caliper sticking on a 2004 Subaru Forester XT

So, I have a sticky caliper on my 2004 Forester XT. 

The symptom is that it will randomly remain locked after I release the brake pedal, pretty much preventing the car from moving, or at least making the wheel drag. It doesn't happen every time I hit the brakes, and seems to happen more often when the hill holder feature gets engaged, but it has happened on level ground as well. If I let the car sit for an hour or so, the pressure will release over time and the car becomes drivable again. 

I've replaced the caliper (all four, actually) and brake line on the one that seems to be sticking. This has not fixed the problem, but it does seem to be the same caliper every time.

The pins and pads move freely. The caliper piston does not seem to be frozen in the caliper- in fact it pushed in fairly easily. So, it seems that for some reason, that particular caliper seems to be holding pressure while the others are not. I don't want to just throw parts at it. I started to think that maybe it was the ABS somehow, so I pulled the fuse and haven't had the problem happen again- but it can be DAYS until it does it again, so I'm just casting a line out here hoping somebody has seen this before.

NASOIC seems to have covered everything I've done so far.

Does the ABS act normally when you activate it? My first guess would have been the line, but since you've done that and checked over the caliper, I'm inclined to suspect a sticking (or more likely stuck) piston in the ABS pump. When's the last time you changed the brake fluid?

Brake fluid was changed and bled when I did the calipers. 

ABS worked normally locking it up on gravel/dirt/leaves every time I checked. It really only seems to not unlock from a dead stop.

After it happened today, I pulled the ABS fuse, so I can't test it at the moment.

Did you replace the hose to the caliper 

Yes, rubber brake line from hard line to caliper is what I replaced.

Check the hard line for smushing or rust....   otherwise it seems like the abs unit might have an issue.  Or something with one of the electronic nannies.  

This seems abs solenoid/valve related to me. 

I saw a similar issue many years ago when the lip of an umbrella type seal was getting stuck inside the ABS hydraulic unit and it couldn’t release until either (1) the passage of time or (2) a little bump of electrons from a diagnostic tool.  Unofficial remote diagnosis:  time to replace the ABS hydraulic unit.

Thanks for the replies. TIme to see if I can find an ABS unit in the junkyard.

I've not driven the car since the last time it happened. I scoured the junkyard today, and they didn't have the one I needed. 

I pulled the ABS fuse, and I may try to see if it happens in a parking lot nearby.

For a 2004 Forester ABS pump, can anybody tell me the difference between parts CO 27534sa001 110 000 41660 and CP 27534sa011 110 000 41670?

Do they interchange?

May have answered my own question: it seems the manual and auto transmissions may have used different ABS pumps.

If anybody can confirm, that would be great.

Or... maybe I found a different root cause of the problem. Apparently the hill holder thing will hold the left front and right rear when it is engaged. Maybe mine is wonky. Since it takes 3 minutes to disconnect this completely. I'd kind of put this off the list, since it happened on level ground before, but for three minutes of work it is worth doing. I think I'll start here before I start mucking about with the ABS system.

I was mucking with it a bit today on ramps in my driveway. It seems like the hill holder is the culprit. It was 80 yesterday, and it is in the low 40s today- holding a socket without gloves for 2 minutes getting the windshield washer reservoir off of the bracket made my fingers numb.

I'll continue disconnecting it tomorrow.  

Final report: 

Something in the hill holder system was getting stuck and caused the entire issue. It took 15 minutes to disconnect and the problem was gone.

There was some cursing involved when I realized that 15 minutes of work could've solved the whole issue without spending so much time and a bit of money on it.

Yay, success. I had a front caliper lock up on my old Impreza. At the time I was heading to the airport. I was around the corner. I figured that I had all-wheel drive and was making it to the airport.

I did. 

Test the wheel ABS sensors.  I think Suby uses the wheel sensors to apply braking as part of their AWD and traction control.  I think it applies brakes to a slipping wheel for traction control.

If the ABS signal is dropping out on an opposing wheel, it might be applying that brake?

I don't know what the Suby computer does when it has an ABS fault, but maybe if you unplug all the sensors and drive it.  If the sticking caliper goes away you've found the problem.  Of course, who knows if the computer will even let you drive it that way.

Excellent advice for one of the newer cars, Curtis.

This is a 2004. There is no traction control. The ABS sucks and the fuse has been pulled for a while now. The hill holder is also mechanical (not electronic like on later cars) and simply appears to have given up after 15 years. 



 

Cool.  Carry on.  Nothing to see here