vacuum issues: hard brakes, no shutoff - PeachParts Mercedes-Benz Forum

WannaWagon · #1 11-22-2005, 01:42 PM

All of a sudden this morning, while pulling up at work, my brakes got hard and when I went to turn the engine off, it would not shut off but slowly idled itself down until finally it stopped. This was the first time I drove the car after a nearly 800-mile round-trip weekend journey in which there were no indications of problems like this.

I assume this is some sort of vacuum leak. But it's strange that it happened so suddenly. Do these two problems together point to a particular problem or place to look for a leak?

#2 11-22-2005, 01:53 PM

It has to be a fairly large leak to affect the brakes. Either that or you have a problem with the vacuum pump. I would start at the pump and look for loose vacuum lines, then put a vacuum gauge on the main line with the branch connections blocked.

boneheaddoctor · #3 11-22-2005, 02:23 PM

Quote:

Originally Posted by Craig

It has to be a fairly large leak to affect the brakes. Either that or you have a problem with the vacuum pump. I would start at the pump and look for loose vacuum lines, then put a vacuum gauge on the main line with the branch connections blocked.

yes.....look closely for cracked line etc...cracked check valve...plastic gets brittle with age...

Do you have a vacuum guage of some sort? if not buy or borrow one.

WannaWagon · #4 11-22-2005, 05:26 PM

Thanks. Yes, I have a vac gauge. So I guess now I know how I'm spending my weekend.

WannaWagon · #5 11-22-2005, 05:47 PM

BTW, how much vacuum should I see on that gauge when I test? What is the "normal" vacuum in the system at idle?

Pete Burton · #6 11-22-2005, 05:56 PM

I got a steady 23" last time I checked.

WannaWagon · #7 11-26-2005, 03:16 PM

Thanks for the quick reply.

OK, I tested like you said and this time got around 20" of vacuum when the gauge was connected just to the main line from the vacuum pump. But the reading was jumping around sharply between 17-23" of vac, and the reading did not increase with engine speed. If anything, it dropped.

Whaddya think?

Biodiesel300TD · #8 11-26-2005, 03:23 PM

If it's jumping around you may not have had a good seal on the brake booster line. Try licking your thumb or one of your fingers and plug the booster line that way. Then you can take your finger on and off as well to test is a few times. What happens then?

Brian Carlton · #9 11-26-2005, 03:57 PM

Quote:

Originally Posted by WannaWagon

Thanks for the quick reply.

OK, I tested like you said and this time got around 20" of vacuum when the gauge was connected just to the main line from the vacuum pump. But the reading was jumping around sharply between 17-23" of vac, and the reading did not increase with engine speed. If anything, it dropped.

Whaddya think?

If you have 20" vacuum with the line disconnected from the booster and the gauge attached to the end of the line, this most likely eliminates the vacuum pump as the culprit.

Now, disconnect and plug all other hoses leaving from the main line. Attach a T to the main line and read the vacuum level again. Be absolutely sure that there are no other devices or hoses connected to the main line.

If the test is done properly and the reading is 10", as you mentioned above, the problem is a leak in the booster. The leak is a decent sized leak and the vacuum pump can't keep up with it. The engine is trying to shutdown with 10" vacuum and I'm very surprised that it will shutdown at all with this low vacuum level. Usually they just keep running.

To condemn the booster requires that everything else coming from the main line be disconnected and/or plugged.

unkl300d · #10 12-03-2005, 02:56 AM

Wierd, My 300D just started with the periodic hard brake syndrome. I had my Vacuum pump rebuilt several months ago (new diaphram) and now it started with the original problem again. BUT there is no problem with shut off (so far).

Anyway, thanks to this thread, I can begin checking the lines etc.

WannaWagon · #11 12-03-2005, 04:33 AM

Followup: Turns out my vacuum pump was shot. I didn't have time to deal with it myself, so I ended up enriching my indy mechanic by about $550 to do the job. Brakes are good now.

#12 12-03-2005, 05:58 AM

Quote:

Originally Posted by WannaWagon

Followup: Turns out my vacuum pump was shot. I didn't have time to deal with it myself, so I ended up enriching my indy mechanic by about $550 to do the job. Brakes are good now.

BTW your engine uses vacuum to shut itself off and this is why your engine slowly idled down and finally shut off...you had weak vac.

Many people forget these are not gas powered cars where you can just shut off the electricity

rmmagow · #13 12-03-2005, 02:24 PM

How difficult is it to rebuild the vacuum pump? This would be on a 617 300D Turbo. Pump housing and all is part of the block no? Are the kits worth the trouble? Thanks.

#14 12-03-2005, 03:27 PM

I put a new diaphragm in the vacuum pump of my #1 '80 300 TD last August. The old diaphragm had a quarter-inch hole in it. Check valves still good. The job was pretty easy after I removed the radiator and fan shroud. All was fine until a couple of weeks ago, when all vacuum functions failed again. I looked at doing the job without removing the radiator, but it doesn't seem possible for me. Taking the radiator out gets easier each time. Ha ha. I found the new diaphragm torn right next to the smaller of the aluminum washers that sandwich the diaphragm. Did I put something in backwards ? The repair kit did not include any diaphragm diagram, and the new diaphragm was a little different from the original, so I took my best guess. Maybe I got something 180 degrees off or in the wrong order. I'm going looking for an exploded diagram, maybe in steaky files...
Any suggestions are welcome. I'll come back and edit here with anything I find.
Repair kit from Fastlane is $17.88. Entire pump over $400. Guess which one I've ordered.

unkl300d · #15 12-03-2005, 06:38 PM

Wannawagon, glad your problem is solved !

re:1979 300D

I just measured the vac of my vac pump from the disconnected end that goes into the booster (with accesory line 'T' blocked) and it reads 30" steady at idle and will go up with acceleration.

I drew 20" vac directly from the booster nipple and it holds 20" fine.

I drew vac from the accessory line and it holds OK. I interpret that to mean that those lines are not leking at least up to the one way valves.

However, I do find engine oil on the inside of the booster's threaded nipple.
Enough to wet the end of my end finger.
Don't know if that is normal or ignorable.

Nonetheless, my brakes still have episodes of hard pedal, and later they go normal. Just recently, the engine shut off has begun to delay twice.

Any commments or opinions?

(the vac pump diaphram had been replaced about 7 months ago.)

Is there such thing as a lazy booster that works sometimes and other times not??

Thanks