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740 Temporarily Frozen Front Caliper?

R-Pow3R3d

Master of Disaster
Joined
Sep 28, 2018
Location
Pittsburgh, PA
Background: Foam cannon and pressure washed the 745 on Monday. Left parked since then until this morning.

Story: When I left this morning, I backed up to turn around and the brakes were very squeal-y. Didn't seem unreasonable since I had sprayed them with water days before. About 3/4 of the way through my ~20 mile drive to work it felt like the car was dragging and then I started getting a shake in the steering wheel that felt like a flattening tire but there was no pulling. Couldn't hear any strange noises. Stopped the car, checked the tires, nothing seemed out of place, so I finished the drive. When I got to work, I went to back into a spot in a gravel lot. I stopped in the loose gravel facing up hill, then tried to move further up the hill and it didn't want to go (tire spin). This is when it occurred to me the brakes were likely sticking. Got out and found the front wheels to be emanating a great deal of heat (both of them). I left it to sit for the day and then came back when it was time to leave. When I got in, I tested it on the gravel and on some asphalt to see if it felt like it was sticking anymore and it wasn't. Drove the whole way home with the windows down and radio off. No signs of problems. Once home, checked the wheels and there wasn't any heat coming off of them.

Question: So, what gives? :wtf:

Follow-up Questions: What should I check? What is the likely culprit? What should I plan to replace in short order?
 
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If it was me I'd get the front up on jack stands, and get the wheels off. I'd put new pads if they look thin...At least remove the pads and press the caliper back and see if the cylinders compress as they should. Put it back together and lube everything properly.
 
Old brake hosing can disintegrate internally, shedding flaps that act as one-way check valves that allow the high pressure fluid from the MC to the calipers but not the relatively lower pressure fluid to return.
 
Old brake hosing can disintegrate internally, shedding flaps that act as one-way check valves that allow the high pressure fluid from the MC to the calipers but not the relatively lower pressure fluid to return.

:nod:
My guess too.
 
If it was me I'd get the front up on jack stands, and get the wheels off. I'd put new pads if they look thin...At least remove the pads and press the caliper back and see if the cylinders compress as they should. Put it back together and lube everything properly.

Pads & rotors were done less than a year ago by the previous owner, so hopefully that's not to blame. I think tearing them down just to verify condition and lubricate is a good plan.

Old brake hosing can disintegrate internally, shedding flaps that act as one-way check valves that allow the high pressure fluid from the MC to the calipers but not the relatively lower pressure fluid to return.

:nod:
My guess too.

Interesting. This sounds like it could definitely be a culprit. I have no records of the hoses being replaced, so that will be on my replacement list.
 
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Line are cheap. Just replace them if you're going to tear it all apart. Probably throw a rebuild kit at the fronts as well. You're going to have to bleed the system anyways if you pop out the caliper pistons for cleaning...

Which front calipers do you have? If you got the dual piston Bendix calipers with the single guide/slide pin, swap them out for the big 940 ones.
 
:-P
Joking aside, one of my 240s did this and it was bad brake lines as said by others. In my case, a small piece of the line made it into one of the calipers.

Seems like a pretty consistent response. If my half-developed plan comes together, maybe I can get these done at the end of the week.

Line are cheap. Just replace them if you're going to tear it all apart. Probably throw a rebuild kit at the fronts as well. You're going to have to bleed the system anyways if you pop out the caliper pistons for cleaning...

Which front calipers do you have? If you got the dual piston Bendix calipers with the single guide/slide pin, swap them out for the big 940 ones.

Yeah, assuming they are original makes me want to replace them regardless of whether they are the issue. I've been on a rampage of replacing anything under the hood that looks worn out. Probably due to flush the brake fluid anyways.

In the few months I've had the car, I haven't pulled the wheels off yet. I would assume the 740T would have the bigger calipers, but perhaps not. I'll have to wait and see.
 
It's a two man job. One guy on the brakes calipers other guy pushing the pedal. Caliper bro pushes calipers back, then pedal bro pushes pedal in and pads go tight at caliper. Do that open close the calipers 40 times, that will fix the sticking caliper problem. Put new brake fluid in first or not..

One side of the caliper pistion at a time if you got two opposing piston calipers. You can take the shoe out and replace it with a big plyers breaker bar screwdriver thing and use that to push piston back real quick. It doesn't take that long with a true bro helping. Work that rod, in, out, in, out.
 
^^

I have done what Simon suggests above and it was successful. You are essentially breaking loose all the crap in the caliper bores and flushing it out. It is time consuming and there is no 100% guarantee of a long term success.

I have had many calipers stick and then release. I doubt this has anything to do with your brake lines. A 240 I fixed up and sold last year did that twice with the same front caliper. The second time it got so hot I could barely get the car to go over 25 mph. I stopped at the car wash and cooled the rotor off and then drove it home using the e-brake only. I replaced the caliper the next day. I've had this happen on several 2-7-9 series cars and it was never the brake lines. Just old calipers that have rust and crud in them from countless years/cycles of use.
 
^^

I have done what Simon suggests above and it was successful. You are essentially breaking loose all the crap in the caliper bores and flushing it out. It is time consuming and there is no 100% guarantee of a long term success.

I have had many calipers stick and then release. I doubt this has anything to do with your brake lines. A 240 I fixed up and sold last year did that twice with the same front caliper. The second time it got so hot I could barely get the car to go over 25 mph. I stopped at the car wash and cooled the rotor off and then drove it home using the e-brake only. I replaced the caliper the next day. I've had this happen on several 2-7-9 series cars and it was never the brake lines. Just old calipers that have rust and crud in them from countless years/cycles of use.

To that end, I'm very much considering just doing the whole deal. Starting fresh. Lines, calipers, pads, rotors. Need to see what condition the pads and rotors are in (should be good since they are pretty new and haven't seen much use).
 
So, this mystery continues... :roll:

I've now replaced the front calipers, pads, rotors, and flex lines (upgraded to SS). Upon replacing all of these items, I went for a long drive yesterday (~40 miles) and everything was perfect. Used the car again today. Drove 20 miles one way and everything was fine. Drove 20 miles returning and in the last few miles I could tell the caliper was starting to drag the car down again. By the time I got home, I could really smell the brakes. I stopped the car on my inclined driveway, put it in neutral, then let out the brakes. It slowly rolled back (slower than a free roll) and the brakes made a horrible, loud, high-pitched squeal. When I parked the car, the front brakes were emanating a lot of heat.

So the question is; now what?! Master cylinder?

Edit: The other thing I should mention is the change in pedal feel. When the brakes were working normally, the brake engagement gradually got more aggressive as you pressed and didn't start to get serious until ~1/2 down. When they drag, the pedal gets very hard very high up in the stroke. Basically, as soon as you start pressing the pedal you are meeting significant resistance, unlike when it's working normally. Hopefully that will be helpful insight.
 
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How fast are you with a jack and a wrench?
If you can catch it when it's really sticking, jack it up, whip a wheel off, and crack the bleeder. If it spurts like a 240-lover finding a pristine grandma-driven 242GT, something is holding pressure in the lines. If you can make it stick again, try cracking the lines at assorted joints all the way back to the master cylinder, watching for the same spurtage. If it just slowly dribbles, then see a doctor.

Being a Turbo, I assume it has ABS? Many more things to go wrong. :-(
 
Yes, ABS. I don't think it would be AB related, but at this point, I have no idea. It's a very curious set of symptoms. At the moment, I'm considering throwing a master cylinder at it to see if that solves the problem.
 
Yes, ABS. I don't think it would be AB related, but at this point, I have no idea. It's a very curious set of symptoms. At the moment, I'm considering throwing a master cylinder at it to see if that solves the problem.

Keep in mind, your abs has valves whose purpose it is to shut off or maintain pressure to certain hydraulic lines, controlling the actuation of the brakes of individual wheels. If the valves are actuating for some reason or acting as one way valves, it could be ABS...
KYP8111.jpg

Hard to know without throwing a bunch of parts at it all.

Just tossing that out there, hope it gets resolved soon!
 
I would check next the play between the brake pedal rod and the brake servo unit.
and after this the play between brake servo rod and master cylinder.

The ABS may be faulty too, still not prone to fail. One of the most reliabale items in cars.

good luck, Kay
 
Keep in mind, your abs has valves whose purpose it is to shut off or maintain pressure to certain hydraulic lines, controlling the actuation of the brakes of individual wheels. If the valves are actuating for some reason or acting as one way valves, it could be ABS...
KYP8111.jpg

Hard to know without throwing a bunch of parts at it all.

Just tossing that out there, hope it gets resolved soon!

The parts cannon is definitely being dragged into service. I'm trying to figure out what choke to apply to the barrel. LOL

At the moment, master cylinder is being purchased and will get replaced.

What part is shown in that graphic? I just looked up the ABS control module, which is $1400 (!!!), but I don't think that's what you're talking about. It looked entirely electrical related.

Edit: Assuming that image represents an accurate depiction of the system, looking more at that it would seem to appear unlikely that two different valves would be acting up and showing the same problem. Still could be, but maybe less likely.
 
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The parts cannon is definitely being dragged into service. I'm trying to figure out what choke to apply to the barrel. LOL

At the moment, master cylinder is being purchased and will get replaced.

What part is shown in that graphic? I just looked up the ABS control module, which is $1400 (!!!), but I don't think that's what you're talking about. It looked entirely electrical related.

Edit: Assuming that image represents an accurate depiction of the system, looking more at that it would seem to appear unlikely that two different valves would be acting up and showing the same problem. Still could be, but maybe less likely.

Over the past 5 years on my 88 744 I've had a similar problem. Perhaps try the following.

Jack the entire front end off the ground. Ensure that the front wheels spin freely / no drag (mine was dragging HARD on the front P/S). Jack the entire rear end off the ground. In neutral / no PB, ensure the rear wheels spin freely / no drag. Although I understand that you replaced the front calipers, be sure to check the rear calipers as well. When you say you replaced the front calipers, what exactly was that? Reman'ed units with all new guide pins and hardware?

IMHO get each of the 4 wheels off the ground and test the caliper for dragging.

Btw, my dragging luckily went away when I put on S60R front calipers / rotors / all SS hoses. Although the rear hasn't been rebuilt, that could still be an issue for me at some point.
 
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