Friend had master cylinder replaced, brake pedal still soft? - PeachParts Mercedes-Benz Forum

Father Of Giants · #1 03-20-2018, 02:37 PM

So she had her master cylinder replaced by a shop and the brakes are marginally better now, what gives? They're very soft, you almost have to go to the floor to get a decent bite.

My guesses are the calipers where not bled properly, the master cylinder was not bled properly, an unforeseen leak appeared, or maybe, just maybe, the master cylinder was bad out the box?

I went in the car with the engine off and pumped the brakes until vacuum boost was lost, the brakes where mushy still.

Then I had her sit in my SDL and did the same to show her how hard the pedal should be without boost assistance. Is this normal in an econoboxes, it's an SL2.

I'll see what's up tomorrow morning.

SonnyMorrow · #2 03-20-2018, 02:55 PM

If your friend had a shop do the work, I would suggest she bring it right back and have them bleed the brakes properly. Of course there could be a leak somewhere, but I would bring it back to the shop who did the work and see what they say. Hard to say over the internet what the problem could be.

sixto · #3 03-20-2018, 02:56 PM

My ‘83 SD did something like that when the brake booster went bad. The symptoms didn’t make sense to me but swapping the booster fixed it.

Sixto
98 E320s sedan and wagon

Diseasel300 · #4 03-20-2018, 03:00 PM

Some smaller cars have crappy feeling brakes from the factory. Think stepping on mushy paper towels. After driving the SDL, the brakes in my Honda drive me crazy and they're certainly not the worst I've felt.

Father Of Giants · #5 03-20-2018, 03:18 PM

We'll see, the brakes might just be normal for what it is.

pimpernell · #6 03-20-2018, 03:22 PM

Quote:

Originally Posted by Father Of Giants

So she had her master cylinder replaced by a shop and the brakes are marginally better now, what gives? They're very soft, you almost have to go to the floor to get a decent bite.

My guesses are the calipers where not bled properly, the master cylinder was not bled properly, an unforeseen leak appeared, or maybe, just maybe, the master cylinder was bad out the box?

I went in the car with the engine off and pumped the brakes until vacuum boost was lost, the brakes where mushy still.

Then I had her sit in my SDL and did the same to show her how hard the pedal should be without boost assistance. Is this normal in an econoboxes, it's an SL2.

I'll see what's up tomorrow morning.

As another poster mentioned, if she had work done by a shop, then she needs to bring it back if it is not satisfactory. The last thing you want to do is start working on the car, fail to fix the problem, and then she brings it to the shop and they have an excuse to say that somebody else worked on the car, and they are not responsible.

leathermang · #7 03-20-2018, 06:31 PM

Yes, take it back to shop...

Is the brake pedal slowly going down when at a stop ?
might be the brake booster....

tyl604 · #8 03-20-2018, 08:18 PM

The master cylinder has two reservoirs - front and back. It could be that they did not fill up both reservoirs or they did not bench bleed the MC.

BillGrissom · #9 03-20-2018, 09:59 PM

Not a bad MC if the pedal doesn't keep sinking to the floor. Bleeding the brakes is requisite when installing a new MC, and one should flush thru new brake fluid every few years regardless to avoid rust, unless using DOT 5 (like me).

jimmypete · #10 03-23-2018, 11:09 PM

Quote:

Originally Posted by leathermang

Yes, take it back to shop...

Is the brake pedal slowly going down when at a stop ?
might be the brake booster....

How can you diagnose the brake booster problem that would cause the pedal to slowly go down when at a stop? I just swapped out my m/c and it's better but still not right. I might do a full bleed again to see if that helps but I don't understand the symptoms and troubleshooting on the brake booster. Will a booster vacuum leak cause the brake pedal to sink at a stop?

Thanks!

Jim

Father Of Giants · #11 03-23-2018, 11:12 PM

Well, I bled it and it feels exactly the same, it no longer goes to the floor like it used to.
I guess that's the nature of the beast.

sgnimj96 · #12 03-23-2018, 11:40 PM

my 85 300td brakes are little spongy and I have bled them several times doing caliper repairs.

I've replaced the MC and booster, then front calipers and 1 rear at later dates.
Always felt the same after the MC and booster. The booster job was weird, had to adjust the pedal rod length because the brakes were dragging so I know there isn't play there.
Rear brake pads may need adjusting. I did that not too long ago with some rear axle work.

The brakes stop well, I was tested them recently as the same question was in my head about the pedal travel vs other cars.

vstech · #13 03-24-2018, 12:32 AM

Ok, the brakes on these cars are extremely simple, and extremely high quality.

If the pedal is mushy, you have air in the system.

If the pedal drifts to the floor, you have a bad master cylinder.

If the pedal is hard to press with the engine running, you have bad vacuum supply, or a bad booster.

That’s it.

If a wheel is dragging, you could have caliper issues, flex hose issues, or rarely master cylinder issues.

tyl604 · #14 03-24-2018, 10:31 AM

Vstech - excellent; that is the most succinct explanation of brake problems yet.

vstech · #15 03-24-2018, 11:43 AM

Yup, and with 4 wheel disc non abs systems, that's it.

Abs puts some kinks to the mess...

Drums are another ball of wax, and don't get me started on proportioning valves or variable valves...

I LOVE the brakes in the older mbs...