Brakes slow to stop the car after pad change - PeachParts Mercedes-Benz Forum

1981_300sd · #1 10-07-2008, 10:31 AM

Hey all,

I got a 1980 w116 300sd recently. The brakes seemed to be a bit weak and made some noise on stopping, screeching noises probably from rust since it was sitting for a long time.

So I decided I would check the whole system out, change all 4 wheel pads, rotors looked good, bled with DOT 4, and installed new rubber lines.

Thought it would brake like a champ now, took it on a test drive, and it seemed to be worse than when I started - the car stops, but takes a long time. Not even near close to safe on freeway, or even back streets for that matter.

I did some searching and some people have mentioned vacuum and brake booster problems, but I don't think it did this bad of a job before I started working on the brakes.

The vacuum door locks do not work, but the blower turns on and off from the climate control panel (eventually, after the car has been running for a minute or so) so the vacuum must be working somewhat right?

Also, I did the brake bleeding with the wife in pumping the pedal while I did the bleeding. Could there have been an issue with her not pushing down far enough - she is short and cant push all the way down to the floor, which is bad any way, and when I opened the bleed screw, fluid comes out but does not "spray" out as hard as I have seen other cars do.

Any suggestions on where to start looking or common causes for this behavior?

Thanks in advance.

79Mercy · #2 10-07-2008, 10:33 AM

the calipers may be frozen.

Actros617 · #3 10-07-2008, 10:49 AM

I too had the same problem brake, what i did is i release the all of the stored vaccume in master brake cylinder and restart the car to rebuild the vaccume pressure and it went back to normal.

mobetta · #4 10-07-2008, 11:40 AM

Power bleeder. I made a HOMEMADE one for almost free.

Here, post #10

winmutt · #5 10-07-2008, 11:40 AM

Brakes firm with engine off? Get any oil on the discs? I recommend getting on an clear and open highway and locking up the brakes once or twice.

1981_300sd · #6 10-07-2008, 12:05 PM

Ok -

"Calipers may be frozen"

How do I unfreeze them?

"release the all of the stored vaccume in master brake cylinder"

Just disconnect the vacuum line to the master cylinder and re-connect / start car?

That power bleeder looks cool - for sure with my other cars - sooner if I need to start troubleshooting possible air in the system issues.

Winmutt - brakes seem firm with the engine off, ill check again for sure when I get home. I will try driving it around some more and try locking up the brakes a few times but maybe not on the highway - too soon to die

vstech · #7 10-07-2008, 03:10 PM

another possibility is the calipers, or at least one of them may have been damaged when you compressed them to put in the pads.
when you wife was pressing on the pedal, is it possible you ran dry on fluid? even once, and you have to start all over again, from the top. possibly even needing to bench bleed the MC. Also, it's been noted here that an old MC when pushed to the floor during the bleed process, you could have damaged it.
please describe the braking action. when you push on the pedal with the car running, does the pedal hold firm? how far can you push in the pedal? is it difficult to push on the pedal?

winmutt · #8 10-07-2008, 03:50 PM

You can bench bleed the MBC in place on the car.

barry123400 · #9 10-07-2008, 04:14 PM

I would jack up one wheel on the front and one on the back. Have someone apply the brakes moderatly and see if you can turn one of the wheels by hand. One section of your master may be out from the bleeding effort. The master cylinder was forced to utilise old unused areas of the bore when bleeding. This can damage the sealing rubbers. Happens. You are also aware it is hard to get fluid into the rear portion of the master plastic brake fluid retention tank unless the front portion is really full.

1981_300sd · #10 10-07-2008, 05:52 PM

vstech -
As for the braking action, the pedal does stay stiff when braking, does not go to the floor. I would say that its not too much different (in stiffness terms) from how it should normally function. The problem is though, that this is not stopping the car lol.

If im going slow, then need to push the brakes lightly, it will slow down / stop the car - feels firm like it should. But say I go like 40, and need to stop quickly, then I push the brakes, and still slowly decline in speed and stop. If I push extra hard, it does stop a little quicker, but still not good enough to be safe.

I halfway wonder if barry123400's post is the issue. I wonder if part of the retention tank did go dry but it still looked full - it was a little dark outside when I did the bleeding - thought I had it full all the time but maybe it was not.

I will try jacking up the car and testing the wheels to see if they stop turning when brake is applied, but I know I did that when bleeding - maybe I was pushing the pedal too hard when testing.

Is there any way to tell if air has entered the system other than fully re-bleeding the brakes?

Thanks.

Actros617 · #11 10-07-2008, 07:00 PM

Dose your brake pedal sinks down when holding?? Here's how to check with the car running press the brake pedal and hold if it sinks and your brake fluid stays at the same spot than there's something to do with the Power Brake Assistance (vaccume). Also dose it take a while to shut off your car???

vstech · #12 10-07-2008, 09:00 PM

grab a hold of the master cylinder. pull on it side to side. does it move? it's possible that it is not sealing the vacuum of the booster. if that's ok, you're going to need a mityvac. it's really sounding like a booster problem.
as mentioned above, is your car shutting off with the key?

tyl604 · #13 10-07-2008, 09:27 PM

Back in the old days we used to rough up the brake pads with sandpaper and then burn them in on the highway; otherwise sometimes they would develop a glaze and not stop very well. Did you perhaps get grease on any of the pads; do you see any glaze at all on the pads?

DubMutant · #14 10-07-2008, 09:51 PM

I had the same impression after doing a lot of work on my brakes. (new rear calipers, rotors, all pads and hoses, new Master Cylinder, new hard line, new fluid)
It took a good week of hard driving for me to feel like the car was stopping better, but it happened. I also has a small vacuum leak to the booster, and after that was fixed, stopping was very good.

I also used more aggressive pads (Mintex red box) them the OE Star Tech. It did take a little bit to break them in, IMHO.

wbrian63 · #15 10-08-2008, 08:06 AM

If I read your original post right you said "rotors looked good" -

If you didn't replace or turn the rotors, you're backing up. The existing rotors will be glossy from use and will not allow the new pads a good flat friction surface - that's why your brakes don't work as good as before the change.

The new pads will eventually "seat" to the existing rotors, but more frequently, they just cook themselves to an early grave, or turn really hard and "squeaky" for the balance of their usage life.

New rubber hoses and new DOT4, if properly bled, is always a good idea.

In all cases with modern disc brakes, you need to "bed" the pads into the rotors. Some advocate a more extreme approach - repeated moderate stops from 50mph down to 10mph, never coming to a complete stop, followed by a lengthy cruise at speed to allow the brakes to cool. Others suggest a more moderate approach - repeated moderate force stops from 30 to 10, but still a drive afterwards to allow the brakes to cool.

Many on this forum suggest that MB rotors cannot be turned and instead should be replaced. Opinions vary as to whether aftermarket rotors are any good - some have had good luck, others have horror stories to tell. Even on my W140, the cost of new rotors from the dealer is reasonable, and I don't have to search around for an "old timey" auto parts store that will actually turn a set of rotors for me. Most of the chain stores 'round here don't even offer that service any more.