Brakes and engine shut off acting up at the same time. - PeachParts Mercedes-Benz Forum

vivi · #1 08-31-2015, 08:07 AM

My 1981 300td w123 has been running great ever since I dug it out of a snowbank 6 months ago. I normally only take it 15mins to work and 15mins home 5 days a week. I dont think I've ever gone so long without even one tiny little problem.

Yesturday I drove it 2 hours on the highway. About halfway through my trip the brake pedal started sinking to the ground. The car still stopped normally buy I had to push down way further than I'm used to. So I pumped the brakes a little and the pedal came up again. But then the pedal was stuck up. When I braked the pedal would only go down a tiny bit and it was way harder to stop the car. I had to really use some force to get the car to stop. When I parked the car, the engine wouldn't turn off so I had to pop the hood and push the lever. The engine not turning off hasn't been an issue for about three years when I replaced the fuel shut off valve and disconnected the leaky doorlocks, and blocked off the vacuume to the engine with a golf tee. Why would the brakes act up at the same time as the engine? On the way home I was sitting at a stop light with my foot on the brake, the pedal was still raised and wouldn't sink when I pressed down, and suddenly I felt the pedal sink down to the normal level and the brakes felt perfect the rest of the way home. When I got home the engine turned off without a problem. What the heck is going on? I am not a mechanic and probably have no buisness owing such an old car, but I'm in love with it so any suggestions would be greatly appreciated!

tyl604 · #2 08-31-2015, 08:48 AM

Definitely a vacuum problem. Sounds like something just failed to work in the vac system for a bit due to lack of use and then it got freed up again. I had the same exact thing happen to my brakes about a year ago when I had only been driving my 81 300SD about five miles a week. Thought I was going to have to replace my master cylinder or maybe brake booster. Then magically the problems disappeared. Has been a year and I have not had it crop up again.

Of course I wait for it to happen again because I never fixed anything.

Mike D · #3 08-31-2015, 08:52 AM

Check the vacuum pump. If you've never replaced it there is a good chance it is getting ready to "grenade".

vivi · #4 08-31-2015, 09:03 AM

Okay great, Thanks! The vac system is actually something I'm pretty comfortable testing myself so that is great news. My vac reserve tank sprung a leak so it isn't even tied into the engine just like the door locks. I have been waiting to go over my whole vac system till I replace or fix the reserve tank. But I've plugged it up with golf tees and the car still gets me to work so I have been procrastinating. I have yet to find a reserve tank for sale anywhere. My brother says he can just plug the hole in mine but I know he will never get to it.

leathermang · #5 08-31-2015, 10:23 AM

check the vacuum first... but....

.... I am going the other direction.... in the ' old days' ... this was the sign of the master cylinder itself having a problem.. If the vacuum is not present then you have extremely hard braking pedal pressure needed and the excess travel of the pedal..
but if only the pedal travel is primary... then the problem was the seals in the master cylinder..

brake pedal occasionally sinks to floor

Master Cylinder – Auto Repair Help

''' Poor internal sealing due to worn or hardened cup seals is most evident by the brake pedal slowly sinking to the floor during periods of light brake applications, such as sitting at a stop. '''

http://www.dummies.com/how-to/content/how-to-troubleshoot-brake-problems.html

'''Does the pedal stay firm when you continue applying pressure, or does it seem to sink slowly to the floor? If the pedal sinks, your master cylinder may be defective, and that’s unsafe.'''

vivi · #6 08-31-2015, 11:02 AM

Hi thanks for your reply!
could you please re-word the part where you say but only if the pedal travel is primary? I'm not quite sure what you mean by that.

I have a little hand vacuume pump. It has been years since I've looked into my vac system and I wouldn't know where to start testing for the root of this particular problem. I will first test the vacuume pump like another user suggested, but I wouldn't know where to go from there. I'm hoping since the engine is struggling to shut off that means it is a vacuume problem. replacing the master cylinder sounds aweful.

also, thanks for all the links. I'm making my way through them, very useful!

leathermang · #7 08-31-2015, 11:13 AM

Quote:

Originally Posted by vivi

Hi thanks for your reply!
could you please re-word the part where you say but only if the pedal travel is primary? I'm not quite sure what you mean by that.
..... replacing the master cyliner sounds aweful.

Sorry, I do not know what it means either .... LOL

I mean if it is the movement of the pedal is the main symptom you are seeing... as compared to EXcessive pedal pressure... the latter meaning you have NO power help pushing the brake pads at the wheels... which means you do not have vacuum help... '
then the old rule was that this symptom was the master cylinder itself..
and changing it is not all that bad... no under the car stuff except the bleeding... but you must follow the rules about BENCH bleeding no matter what others will say about that... follow the rules on the new master cylinder package...

http://www.peachparts.com/shopforum/diesel-discussion/343341-how-difficult-replace-master-cylinder-1979-240d.html

vstech · #8 08-31-2015, 11:19 AM

while, vacuum problems are certainly part of your issues, either the checkvalve, or the pump and lines... it's not your only problem

TOTAL failure of the vacuum system will not cause your pedal to sink low and have low braking, and the need for you to pump the brakes to get pedal back.

I think, master cylinder, and booster issues are showing as well as vacuum problems.

the booster could be leaking, or filled with fluid, and your rapid pumping may have depleted the vacuum reservoir.

uberwasser · #9 08-31-2015, 03:00 PM

Agreed, a lot of people seem to mix up soft pedal and hard pedal.

Hard pedal generally means lack of vacuum assist.

Soft pedal means air in the brakes/fluid leak.

Since you're experiencing both I'd wonder if your master cylinder's rear seal wasn't allowing brake fluid to leak in to the brake booster. That's what vstech was getting at I believe.

Have you checked your brake fluid level since this occurred?