1985 300TD strange brake behavior - PeachParts Mercedes-Benz Forum

azitizz · #1 03-15-2016, 12:15 AM

My brakes have started feeling odd lately. The symptom is when I would push on the brake after some normal driving its fine, but if I push it again immediately, like pumping it, the subsequent times the brake is not very responsive and feels tight.

Its as if it doesnt want to go down as easily and doesnt brake as effectively after that first brake. It returns to normal again for that one brake after driving again some few seconds or so revving the engine perhaps.

Another symptom, perhaps related but could be unrelated is that the engine is now running longer after turning the key off. Sometimes it wont shut off at all, but I just step on the gas and let go quickly and it kills the engine.

Seems this problem started appearing more what the brakes were getting worse, but not sure.

Vacuum system? It my guess, but any thoughts? Any particular part of the system? Its almost never been fully functional. I had the locks actually working last summer I believe for the first time but didnt seem to last.

BWhitmore · #2 03-15-2016, 12:28 AM

Sounds like a vacuum pump failure.

Rogviler · #3 03-15-2016, 12:31 AM

Yeah, definitely a vacuum issue. I have a very small leak somewhere, such that only once in a great while (a few times a year maybe), if I sit with my foot on the brake for a long time or hit every red light through town it starts to shift harder and I have to let it idle for a few seconds before it'll shut down.

You'll probably just have to start isolating systems with a handheld vac and see what won't hold it.

-Rog

fezcabdriver · #4 03-15-2016, 03:58 AM

It might all be related with the brake booster being the culprit.

However to isolate the inability to shut off car, try turning off the car in either the door locked and the door unlocked position. Or just get under the hood and disconnect the yellow lines from the 4way and plug it with a golf t. If that works, you have a leak in the central locking system.

But again, your braking issue also sounds like a booster issue which is also related to the vacuum system.

superapan · #5 03-15-2016, 07:32 AM

I have the same problem. Brakes are sometimes weak, parallel parking can get a bit scary. Sometimes engine doesn't shut off. The Mrs hates much about the car, but these are probably her biggest issues (aside from the water leaks, manual glowing on a switch, rough idle, leaking fuel tank, oil leaks...)

I figured it was a vacuum problem, and I've bought a hand pump tester on Ebay, but haven't gotten around to do it just yet.

Rick76 · #6 03-15-2016, 07:58 AM

Possibly the pump, however there can be many sources of leaks that must be ruled out first as other posters have stated.
I bought a new pump a couple of months ago from MB. $413 list Canadian.

toomany MBZ · #7 03-15-2016, 09:48 AM

There is a check valve between the pump and line going to the poster.

First however, you need to determine how much vac is being produced. Connect a vacuum gauge to one of the orificies with the other blocked off, then check the other. You should get ~22 inHg on one and ~15inHg on the other.

Zacharias · #8 03-15-2016, 10:24 AM

What toomany said.

Also check all the vacuum lines and the rubber junctions and the main vacuum line and its ports for splits or other damage. This may sound like an obvious step but make sure you have lots of light on the bits when you do it, and make sure you check each piece 360 degrees.

I had a low vacuum issue two years ago and found that one of the junctions had split on its underside, so it wasn't obvious, causing a major leak. Then I also realized that all the junctions were so old they weren't sealing properly to start with.

Rogviler · #9 03-15-2016, 11:44 AM

An easy way to test vacuum systems is to blow smoke into the lines. If you already smoke that's easy, but a $1 cigar from the gas station is perfect. Just blow it in and look for the smoke. If it's in the lock system then you probably won't find it that way is all.

I've noticed that on this board the most expensive things are often suggested first (Runs rough? You gotta pull the engine and buy a brand new one!), but seriously, look at the easy and cheap stuff first.

-Rog

BillGrissom · #10 03-15-2016, 02:20 PM

There must be a gazillion posts on vacuum issues, booster problems, and ways to check, plus rebuilding the vac pump or alternative electric pumps. So, best to read those first then ask specific questions. I solved all my vac problems using these, and haven't found any vac issue that would make me give up on the car. I sure wouldn't pay the $400 price of a new pump.

Diesel911 · #11 03-15-2016, 02:38 PM

Why not check the Vacuum with a Vacuum Gauge. I beleive you can get them as a free rental from places like Autozone. They are usually a combination of Vacuum and Fuel Pump Gauge in one gauge.

When I had a similar symptom it turned out the Check Valve the srews into the Vacuum Pump was all gunked up inside. Sprayed it out with WD-40 and everything went back to normal. The Check Valve is where the main line screws into the Vacuum Pump.

Some people have removed the Vacuum Pump Check Valve and found all of the parts had fallen out of it. Also don't use brake cleaner on it as it has plastic parts inside and brake cleaner messes with some plastics.

Also it does not cost anything to take a look at the Check Valve.

vstech · #12 03-15-2016, 03:40 PM

Ok... DO NOT DRIVE THE CAR until you find and fix the problem.

Aside from the stopping problems. .. if the pump is the issue. .. it WILL TAKE OUT YOUR ENGINE !

azitizz · #13 03-15-2016, 11:42 PM

Quote:

Originally Posted by Diesel911

Why not check the Vacuum with a Vacuum Gauge. I beleive you can get them as a free rental from places like Autozone. They are usually a combination of Vacuum and Fuel Pump Gauge in one gauge.

When I had a similar symptom it turned out the Check Valve the srews into the Vacuum Pump was all gunked up inside. Sprayed it out with WD-40 and everything went back to normal. The Check Valve is where the main line screws into the Vacuum Pump.

Some people have removed the Vacuum Pump Check Valve and found all of the parts had fallen out of it. Also don't use brake cleaner on it as it has plastic parts inside and brake cleaner messes with some plastics.

Also it does not cost anything to take a look at the Check Valve.

I think that will be my first look. By teh vacuum pump do you mean the part that says "Girling" on it, Round black thing against the firewall. I thought this was the brake booster, but I guess they are related no?

t walgamuth · #14 03-15-2016, 11:52 PM

No the vac pump is on the front of the engine on the drivers side of the car. Its about the size of a large aluminum grapefruit and is driven off the cam chain.

superapan · #15 03-16-2016, 10:29 AM

Quote:

Originally Posted by vstech

Ok... DO NOT DRIVE THE CAR until you find and fix the problem.

Aside from the stopping problems. .. if the pump is the issue. .. it WILL TAKE OUT YOUR ENGINE !

Could you please elaborate? Besides the stopping problems, how can it take out my engine?