Brake problems, and question - PeachParts Mercedes-Benz Forum

woodrat · #1 06-12-2011, 11:36 AM

My 77 240D has an intermittent problem of one or the other front calipers sticking and dragging. It's not all the time, and it's not the same one every time. When it's dragging really bad the pedal feels really tight and high, and sometimes if you jab it really hard the brakes will return to normal. For these reasons, I decided I would try changing out the master cylinder first, but looking at the cylinder and the VIN, I realized that mine has the extremely expensive pressure sensor model, that is going to cost something like $285, instead of under $100 for the other one.

What exactly does this sensor do, and can I get away with buying the non-sensor MC instead, and just leaving the wire hanging loose? Is it still possible to find rebuild kits for these? Assuming that mine is not horribly pitted inside, I'm perfectly capable of honing mine out and putting new seals in it, for way less than $285.

JimSmith · #2 06-12-2011, 12:01 PM

Brake caliper sticking and dragging are caused by the mechanism that pulls the piston in the caliper, and thereby the brake pad, back away from the disc fails. This mechanism is the square cross-sectioned sealing ring between the piston and caliper bore. Once this rubber ring wears too much there is nothing left to pull the piston back, so the brake pad stays in contact with the disc, generating significant heat. This heat gives your brake pedal some of the peculiar feel as the fluid expands.

The fix is two new or remanufactured calipers. There are many who attribute this condition to the rubber hose as well. So, if you change the caliper you should also likely change the hose that jumps from the hard line on the suspension to the caliper.

Jim

scottmcphee · #3 06-12-2011, 12:02 PM

Left or right caliper intermittently. Maybe change out your brake line hoses at each wheel first before master cyl, do a fluid flush. Sounds like deterioration of hoses on the inside.

junqueyardjim · #4 06-12-2011, 12:05 PM

Well, I don't think the problem is in the MC. If your calipers are sticking they are usually the cause. Perhaps for a very low price you could find some used calipers. Even rebuilt calipers are not very expensive.

woodrat · #5 06-12-2011, 12:12 PM

I can get calipers alright, I just was thinking that the way they sometimes worked fine maybe meant that the problem was elsewhere. Thanks for the help! I'll change out the hoses, too, they're probably pretty old.

Biodiesel300TD · #6 06-12-2011, 12:27 PM

This could be a vacuum issue as well. If you loose your vac you get hard brakes as well. Try hooking a mity vac up to you main vac line. And with the car running push the brake pedal firmly but quickly a couple times. The vac will drop off to 0 or near 0 after a few pushes of the pedal. But it should build back up with in a few seconds. If it takes more than a few seconds to build then you've got a weak vac pump.

This is what happened in my 240. If I used the brakes three times in a short interval the third braking would have no power assist and the brakes where really hard and had little stopping power. But if you waited a second or two before applying the brakes the third time it was fine. I swapped the vac pump and the problem was gone. I tested my pump as stated above.

This may not solve the issue but it's free to check before you start throwing parts at it.

Yak · #7 06-12-2011, 12:46 PM

The FSM brake re-build sections for the W123 brakes show the piston retraction "mechanism" to be vili-like fingers on the piston/caliper seal. The pressure moves the piston out and the rubber fingers flex it backwards. Replacement seals are a square cross-section ring as Jim mentions in post #2. The concept is the same where the square is torqued slightly out of shape by the pressure and when the pressure releases the rubber square pushes the piston slightly backwards. As the pad friction material wears, the extension-retraction is sort of self adjusting.

If the calipers (or sealing rings) are old or original, the retraction ability might be reduced. The intermittent nature may be dependent on some other variables as well - age/condition of the fluid and presence of moisture, how hard you've just applied the brake, stop and go driving, etc.

Re-build kits for calipers are cheap, and if you're familiar with boring/honing an MC it sounds like you'd be comfortable rebuilding a caliper. The rebuild would also give you the opportunity for a good flush, new hoses, etc.

<> To clarify: boring/honing are not part of the caliper repair. The comment was included in reference to the OP's assessment of his skill set. A caliper repair is probably not a task for a beginner. On brakes, you should have some judgement or experience to know when to give up on repairing the old and replacing. On brake pistons and bores, any pitting probably falls into the "horrible" category.

JimSmith · #8 06-12-2011, 02:09 PM

For the most part, the operation of the caliper bore/piston OD and fit of the seal depend on unbelievably precise dimensions. Honing to remove more than light surface deposits is not recommended. The kit for "rebuilding" consists of mostly new rubber parts and it is cheap. So, IF you tear down the caliper once it is out, and there are no deep scratches or pits in the working surfaces of the bore, you can clean things up without changing any dimensions in the critical areas and replace the rubber seals and parts and you will likely have a good caliper. I have done this on several occasions with success.

If there is any scratching or pitting, brakes are safety items, and the consequences are severe if there is a failure - buy professionally rebuilt calipers. The rebuilding process involves removing plating, reapplying plating, and several machining operations to arrive at the very precise fits needed. This is not what you are going to do at home. So don't get started down the path of material removal.

Jim

carnut · #9 06-12-2011, 04:34 PM

I dont know how pratical this is for you but if a wheel is dragging, jack up that WHEEL AND CRACK OPEN THAt calipers bleed screw. If the wheel quickly frees up its not the caliper but the hose to it. Change both and flush the system.

woodrat · #10 06-12-2011, 05:55 PM

I've kitted calipers before, and honed many brake and clutch masters, clutch slave cylinders and brake cylinders, so I'm familiar with the process. I get that you can't hope to hone out any more than the lightest pitting or scratches without getting into trouble. In this case, though, I would go ahead and just buy rebuilt calipers, since they don't cost that much and my time is worth somewhat more than nothing, to me, anyway. For the cheaper master cylinder, I would just buy that, too. But when they add a little extra port for a sensor, and then nearly quadruple the cost of the cylinder, I start thinking about rebuild kits or using the non-sensor master cylinder. There can't be anything about that sensor-using master cylinder that should make it cost that much more than the other one.

thanks!

woodrat · #11 06-12-2011, 05:56 PM

Quote:

Originally Posted by carnut

I dont know how pratical this is for you but if a wheel is dragging, jack up that WHEEL AND CRACK OPEN THAt calipers bleed screw. If the wheel quickly frees up its not the caliper but the hose to it. Change both and flush the system.

That's a good idea, thanks!

woodrat · #12 06-14-2011, 01:18 PM

Just so no one is thinking that I decided to repair my own calipers with home made o-rings and sandpaper or anything like that, I would like to add that I went ahead and bought all rebuilt calipers and soft hoses for the car, and I'll be installing those tomorrow.

I'm still interested in my original question about the master cylinder, though. In case it also needs replacement, what does that sensor do, and can I get by with the non-sensor MC if it comes to that?

Thanks!

leathermang · #13 06-14-2011, 01:35 PM

Woodrat...we are proud that you take your safety seriously... we , and you , will sleep better for that...
How about a picture of that MC in question... if that sensor is in the lid... I do not think you will have to have that on the replacement...although it is a nice feature...
If the sensor is somewhere else I do not know what it is for....

woodrat · #14 06-14-2011, 01:56 PM

The sensor is screwed into the MC itself, near the end.

leathermang · #15 06-14-2011, 02:02 PM

I assume yours is a 115 ?
I only have 123 manuals.... and I do not see anything except a type one and type two brake fluid warning system...
but that seems like a bad place to have that warning sensor.... since you are already in trouble if you get down to air there in the mc tube.......

I wonder if it could have to do with setting the adjustable rod between the MC and the Brake Booster ?
If that is set too long then you do not get full action of the MC...