After a holiday of two weeks and leaving my 850 standing in the rainy Dutch wheather, I have a bit of a problem with the breaks: The front left break disk heats up disproportional,and the wheel jamms slightly.

After the holiday,at first the car would not move. I never use the handbreak, so that was not the problem. I was ready to get out of the car to see if someone had blocked the wheels, but tried one more time. The car kind of "broke free". Immediately after, when applying the breaks, they felt shaky. Kind of on/of, but the very fast. I never had the feling I could not break. Maybe it was not on/off, but 50-80% intermittendly break power.
I blamed this (and the car being stuck) on my presumption that the disks probably were corroded heavily except for the spot were the pads had rested. I thought this problem would go away and indeed it grew less and less. After about 100km it was gone.

After about two weeks with several short trips and two longer ones (100km -500km) with no indication of any problems, it was by coincidence time to change tires. The mechanic then noticed that the front left wheel was kind of stuck and that the disk was quite hot.
After the disk cooled down, the wheel could (as far as I could see) rotate without extra effort. Also when driving at whatever speed, I notice nothing in steering breaking, etc. However, the disk of the wheel in question does heat up more than the others (tested by appying some water on the disks).

Now the big questions: What is it and how can it be helped?
I it slightly stuck calipers?
Has the disk bended?
Excessive build up of break dust?
other suggestions?

Looking forward to your insights.

Yes, it could be a sticky caliper, but if this is the case, you will have significantly more pad wear on that side, more than likely evidenced by much brake pedal dust. A brake line that is collapsing can also keep your brakes on somewhat, so don't just assume caliper. I've had people come in after having brakes done somewhere else, and the brake line is wrapped up in the suspension. It is also possible that the hub bearing is compromised internally, and if the grease in this bearing has become thin, it will also heat up...but this should be accompanied by a whining, and/or droning/grinding noise. If the caliper is sticky, you can exercise the caliper, apply brake grease and move the piston in and out.

Chris

Yes, it does look like a sticky caliper then: There is more break pad dust. What is a collapsing brake line (I assume the one with the hydrolic fluid)?
There is no noise while brekaing, so I do not think is is the hub bearing.

I will go with exercising the caliper (removing pads, apply break somewhat, grease tem, try to push pistons back and repeat). See what happens and the share my expreriences.

Thanks for all the help so far, keep you posted.

It sounds like one of the pistons is stuck and holding the pad against the rotor. This was probably happening before your holiday, but not noticed. Have a technician remove the pads and lubricate the pistons on the bad wheel. It is not a difficult task.

Many times, my car will sit for a week at a time in the rain and the rotors will get rusty. They clean themselves off at the forst or secons stop sign. Never should your wheel get "stuck" to the rotor.

Klaus
--
95 854T, 88 780, 88 245

Shouldn't I have noticed that the pistons were stuck and holding the pad against the rotor. Even on the longer drive I notice nothing. I am not questioning your diagnosis, but just wondering what I should/could have noticed also to prevent the in the future.

With what can I lubricate to pistons. Grease, WD40?

After talking to my mechanic he said to inspect and clean off the slides under the pads. Also to get the brake fluid replaced and bled properly, any water in the line will compromise the pistons. You can use brake grease on the pistons if they feel rough (shouldn't) and make sure the slides are clean without rust. You should be able to spin the tire while it is on the jack for at least one revolution.

Klaus
PS Mechanic also agreed that rotor rust is normal.
--
95 854T, 88 780, 88 245

I would guess that the calipers need cleaning etc. I would suggest NOT putting cold water on a hot disc. Great way to warp them.

Thanks for your input. Won't put any cold water on them again.
But how does one clean the calipers etc?

You realize that a hot disk exposed to cold water will NOT hurt the disk. If that were the case, we could never drive in the rain again! :-)

I think my 1988 780 has your problem also.

Klaus
--
95 854T, 88 780, 88 245

Of course you are right, but I understood the warning not to put any cold water on just a part of the disk. I presume that driving in the rain has more spreaded cooling down effect.

If the caliper stick they can sometimes be loosened up by pushing the piston back into the caliper step on the brake pedal to extend the piston, repeat a number of times. be carefull not to damage the pads.

Well, here is the update, and some new questions.

I found an excellent guide on the break section on http://au.geocities.com/ozbrick850/Brake-rotor-johnsguide.html ) which I used in combination with the Haynes manual.

First I took of the weel, the little spring and the guide pins. The sliding off the caliper proved to be quite difficult, probably because the caliper was stuck.

Once off, the piston was quite dirty. The rubber over the piston was torn and seemed lose. My first quesiton:
I am not sure how it is supposed to be, over the caliper on the inward side, or between the piston and the caliper. Since ist was halfway either and torn and since the piston was dirty I decided to take it off. I then applied the break a little bit, moving the piston out. This worked. I again cleaned the piston, wich proved to be a bit rusty on some spots. I think this might be caused be the torn rubber, and might cause the caliper/piston to be stuck. To push the piston back was quite difficult. With the improper use of a screwdriver as leverage tool, I managed to push it back. I now think a glueing clamp /cramp would be a better way to force the piston back, but this worked. I repeated this several times, in the end it went easier than before, but by no means easy.
Second Question, how hard should it be to push the piston back?
After the I remounted the breaking pads. Despite the jamming breaks, the wear had not been such that I think they needed replacement.
Third question: how thin can.may the pad be?

After all this I am bit reluctant to drive the car, as I am not sure I did the right thing with the rubber between the piton and the caliper. I now put it just between the caliper and the piston, but it is still torn. Is it a part that can be individually replace? I am going to call the volvo dealer to find out. I will let you know.

Anyone who endure the read, thanks. Who supplied me with help, even more thanks. And the ones who can help me with the new questions....

I phoned the dealer, and believe it or not: the rubber dust thing costs a staggering 38 euro (roughly 45 USD)! That is the most expensive rubber ever.
I'll take my chances and not replace the rubber. If needed I'll replace the caliper completely (by a used or refurbished one).