Rear caliper seized

[LNM]

Only one caliper is active on the rear of the 190. Suspect this is where the thudding is coming from under medium braking now (used to be only hard)

The nearside disk is rusty and pitted; clearly no braking been going of for a while there.

I'm going to fit new disks to it anyway, but was wondering whats involved in un-seizing the caliper ? Like do I have to bleed everything ?

Ta in advance for any advice chaps and chapettes.

 

[cheesyget]

Depends on what's seized - it could be the pads to the caliper or to the pins in which case no bleeding required. Or it could be the piston(s) in which case bleeding is more likely to be required. If it is the piston(s) and you were really lucky you might be able to get them unseized and cleaned up without the piston coming all the way past the seal, and therefore not have to bleed, but I've only ever done this on a bike where you can operate the brake lever more accurately and also see what's going on with the piston at the same time. Obviously you can do a more thorough job on the pistons and seals by taking the whole lot apart but this does mean having to bleed everything up afterwards. As a matter of interest, when I collected my W123 a few weeks ago it hadn't been used much for a few months and by the time I got home (only about 8 miles) both offside brakes were binding quite badly and this turned out to be pads corroded to the calipers. A quick clean up and a bit of copper grease and everything was fine. I did discover that all the discs are pretty much worn out and the rear pads only touch about 25% of the disc on the inside but that's a job for another day!

 

[Mikesmerc]

Have dismantled a few 123, 112 and 124 rear calipers that were siezing but usually the pistons stay stuck out rather than in, leading to the disks heating up. Bleeding the brake system is the easiest part of the job, the most difficult part is getting the pistons out. What I used do is remove the pads from the caliper while leaving the hose connected, press on the pedal and try and coax each piston out the same amount(two person job), that is holding the free one back with a pad or something similar, with enough pedal pressure the other one should budge.
Remove the caliper and push the pistons out fully, disconnect the hose,split the caliper (watch the two little 'o' rings)and remove the seal from each half of the caliper, a feeler guage works a treat for this as it has no sharp edges. You will probably find that it is a combination of gunk on the pistons and rust just below the dust cap on the caliper that is jamming it up. Very light abrasive paper to clean the caliper up. When all surfaces are clean replace the lot and you will find that the pistons glide back into the caliper.
You should replace the seals and dust caps with new ones.
If you have any doubts about doing any of the above stay well clear and get someone competent to do the job for you as you cant take chances with brakes or steering for that matter.
Regards
Mike

 

[Richard Murray]

Seized Piston/Caliper?

If the piston is the culprit, after it's been removed from the caliper - do as Mike suggests and remove any surface rust by lightly sanding. After cleaning it up - inspect if it's badly pitted or scored in which case you may need to renew the whole caliper unit. If it's ok, prior to refitting - you can also apply a little Lockheed 'Rubberlube' red grease which is suitable for hydraulic applications. Renew Rubber dust cap and 'o' ring, etc.

Regards

Richard

 

[SUE.]

By the time you strip down the old caliper and buy seals and bits it is sometimes better to replace the full caliper.