BRAKES w116 300sd 1979 - PeachParts Mercedes-Benz Forum

xaliscomex · #1 11-07-2011, 04:35 PM

Own a 300sd 1979(w116) and yesterday I noticed something weird. I started my car and while it was warming up I left my foot on the brake pedal for about 1 minute. As I began to shift from Park to Reverse the brake pedal went straight to the floor. Shocked, I immediately pumped the brake 2 quick times and the brakes grabbed on again. I then waited a few seconds, without my foot on the brake pedal, and proceeded to back up without a problem. I drove 7 miles home without any brake problems and then tried to replicate the same steps at home but without turning the car off. I think I might want to repeat all the steps and shut the car off and then start it again. Just wondering if anyone has ever experienced this situation. I replaced the brake pads and rotors 2 months ago and was driving the car twice a week before the incident. Any advice on what I should be checking for? Do I have a vacuum leak, master cylinder problem or brake booster issue? Im going to bleed the brakes to see if I have any left over air in the line.

Thanks

dude99 · #2 11-07-2011, 04:45 PM

A pedal going to the floor means either you have air in the lines (a compressible substance) or that the fluid is going somewhere it shouldn't. Since yours was a sudden loss of pedal I'd lean more toward the fluid going somewhere. Have you noticed a drop in fluid?

kerry · #3 11-07-2011, 05:03 PM

That's a typical symptom of a failing master cylinder.

vstech · #4 11-07-2011, 05:06 PM

Quote:

Originally Posted by kerry

That's a typical symptom of a failing master cylinder.

if there is no fluid loss, and the brakes feel normal now, yup.
change it now, before you can't pump it up.

xaliscomex · #5 11-07-2011, 05:28 PM

No fluid loss. Only a few cracked vacuum connectors that i just replaced 5 minutes ago. You know, about 3 months ago I remember traveling around 45 miles per hour(mph) and had to come to a stop a quick stop. I pressed the brake pedal and then all of a sudden it traveled to the floor, and I mean traveled to the floor without any force against it. I freaked out and pressed on the brake pedal again and it grabbed. When I got home I checked the front brake pads and I swear to you such brake pads were super thin. There was practically nothing in them. I recall 1 year ago removing the brake pad sensors because rain water was somehow making my brake light go on. Looking back, that was probably a good indication that my brake pads were running thin. Well after the 45 mph incident I replaced the front pads and front rotors. Did not bleed the brakes thinking that it did not need it. Looking back, the brake pads being so thin might of caused the caliper piston to over work, I guess. Not sure, but I'm sure something is related to my previous brake pedal traveling to the floor. I remember another incident where I was driving down hill and had my foot on the pedal for a good while before I stopped. The pedal traveled to the floor and I remember just braking again and it worked. Wow...too many close calls...Not driving the car anymore until I figure out whats going on...

Skippy · #6 11-07-2011, 10:34 PM

Replace or rebuild your master cylinder. The symptoms you describe are consistent with the internal seals of the master cylinder failing.

kerry · #7 11-07-2011, 10:41 PM

And your front pad wear indicator light coming on when wet is how they work. A little more conductivity provided by the water activates them.

xaliscomex · #8 11-07-2011, 11:27 PM

Will replace the Master Cylinder and then post update, thanks for all the replies.

westchester aut · #9 11-07-2011, 11:47 PM

I have a 1979 300 sd my self can I see picture of your car please.I also need parts can you guide where i can find them.Thanks

vstech · #10 11-08-2011, 10:21 AM

I have had similar results from TOTALLY gone pads on the rear calipers.
I'd TOTALLY recommend a new MC, but be certain the pads are thick enough to allow the MC compression.
My 87 wagon had "pedal to the floor before it'd grab syndrome". replaced the MC, no change. TOTALLY FLUSHED entire brake system... no change... replaced the rear pads and calipers, and POW! full solid pedal again.
no fluid was leaking.

Skippy · #11 11-08-2011, 04:13 PM

I did get a similar result on my motorcycle without a master cylinder being bad, but there was more to it than that. Last month I took it to a shop to replace the rear tire. They did that, but the tech who put the bike back together managed to leave out the inside rear brake pad. Despite the missing pad, it still worked so amazingly well that I didn't notice until the steel rotor had worn through enough of the aluminum caliper that the caliper pistons ran out of travel. The service manager seemed a bit embarrassed when I brought the bike back.

@westchester aut: This site sells parts. Go to the top of this page and click where it says "Buy Parts".

Dieselboy740 · #12 11-11-2011, 02:16 AM

My sd did the same thing I put it off for awhile but finally got tired of sitting at a light and randomly having my pedal start going to the floor.

I replaced my master cylinder and that fixed it.

xaliscomex · #13 11-30-2011, 11:43 PM

Replaced Master Mylinder(MC) and seal on Monday, re-positioned the brake reservoir directly on top of the MC with new grommets. I then attached my motive brake bleeder, without adding any brake fluid, and pumped it a few times when I suddenly noticed fluid coming out of the contact inserts caps. I looked a little closer and noticed cracks on each one. The reason brake fluid was not added was because I was going to first check for leaks in the system. The motive brake bleeder has a small pressure gauge that you can read to see any pressure drops. I don't know for sure if I would of caught the brake oil leak if I would have used the old school foot pump bleeding method because I normally leave the brake reservoir cap off when pumping. I normally use the motive brake bleeder on my Chevrolet and on my Civic. First time using it on my MBenz. I did easily remove the contact insert caps(2 caps on the reservoir) and noticed small plastic tubes that move freely up and down. I removed them only because they were already cracked and leaking. Is it normal for these capped openings to release brake fluid when pumped using a leak free adapter with air bottle that pushes air or is it because of such contact insert caps being cracked that brake fluid was able to escape and leak out. I ordered a new reservoir due to the fact that brake fluid level was hard to see anyway. I'm just hoping that brake fluid will not leak out of the new MC contact inserts..Any experiences or suggestions are more than welcomed.. 1979 300sd w116

vstech · #14 12-01-2011, 01:15 AM

the rubber caps can be replaced. phil, can order them for you. (click buy parts at top of page) give phil a call!

xaliscomex · #15 12-01-2011, 08:40 AM

My new reservoir comes in tomorrow and it comes with the caps but I will make sure and keep to keep the old one and order some new caps, thanks. I did not know you could order caps.

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