Screwy Vac.
 

When parking in a garage the other day I lost vacuum. Brakes went, no shut-off valve... It happened again today. Driving slow and hunting for a spot - pull up, reverse into spot - brakes stiffen, here comes a parked car towards me... Isn't there a vac. actuator of some sort on the tranny? Since this has never been a prob. at lights or in traffic, should I start with vac. connection to tranny?

mccan,

I think the automatic does employ vacuum for some control functions, but I also thought the vacuum system had some check valves to make your situation less likely. It would seem you have lost one of the check valves as well as generated a pretty severe leak, probably associated with the vacuum controls of the transmission.

Out of curiosity, does the door locking system still work? I would be reluctant to drive the car if the brake boost drops out intermittently.

Good Luck and I hope someone speaks up to help. I think the parts for this system are inexpensive, but they are a pain to figure out and get to at times. Once again, good luck! Jim

It only happens in reverse so I assume it is not in the pump but in somehting attached to/controlling tranny.

The only vaccuum to the tranny that I can think of would be the modulator which has nothing at all to do with reverse.

If the linkage to reverse is hitting the vaccuum tube and disrupting it, maybe...

If you lose brake vaccuum, wow that would be a hellofva leak.

I don't know why it would only happen in reverse:confused:
Is this coincidence?

It is easy to check for vaccuum with your finger. Take off the small line at the T at the big line to the brake booster and feel if there is some suction. Engine running of course.

If you own an old benz, you MUSST own a Mighty Vac.
This is now a law in this country as well as Germany.

What is this Mighty Vac. of which you speak?

Ed, what do you mean the modulator has nothing to do with reverse.... when you get your car up to about 50 mph in reverse the modulator is what shifts you into High Reverse... you could overspeed your engine otherwise... but you must be sure your steering is tight otherwise the car gets squirrely about 65 in reverse , but this is something that should be reserved for professional drivers, particularly if they are experiencing the same vacuum outage as Mccan.... Greg

I think I saw some toothless yayhoo try that on Cops.

mccan:

You can buy the Mity Vac tool at many auto parts stores for about $30 for the basic tool with accessories - the deluxe version with a hard case sells for more. In addition to checking vacuum, you can check the M-B climate control system, bleed your brakes, draw other fluids from places that are unreachable otherwise, engine diagnosis (gas engines, not enough vacuum on diesels), and more.

Good Luck!
Tom

Check the condition of the main vacuum line -- putting the car in reverse changes the direction the engine "torques", and you could be opening a major leak when you do so. This has nothing to do with the tranny, you can't loose vacuum through that little hose.

You can lose vacuum from a broken plastic part, where the tranny line connects, that opens up with you put the car in reverse and the engine moves over a bit.

Peter

Thanks for the tips. I'll probably pick up a mighty vac. this weekend. I used a $1.99 hand syphon to pull out tranny fluid after I had put too much in. Kind of sloppy.

I took the car out and coasted around the neighborhood. I found that when I kept pressure on the brakes, the vac. slowly decreases until gone. Reversing into parking space was apparently just timing coincidence. Vac. pump seems to be pulling well, but the only check I've done is to put my finger over the main vac. line when car is running. It seems that maybe Vac. Pump is dying and not able to handle the load when brakes make a long constant pull. Or, maybe the brake booster is on it's way out.

mccan

Check to see if there is a one way valve at the brake booster. They are usually replaceable. It should be at the end of the vaccuum line where it goes into the brake booster.

I am too cold to go out and look at mine!

We had snow down to about 500 feet elevation today. What a weird sight to see snow capped mountains in San Jose, CA.

mccan,
I've had the same problem on my car. In the beginning I'd loose vacuum in reverse, a few weeks later I'd loose vacuum as you say when applying the brake slowly. The situation was also worse when it was colder outside especially in combination with the car first being started. Over time it took longer and longer for the car to shut off till it would only shut off manually. Both times I had a torn vacuum pump diaphragm. First time - age, second time - my fault. I replaced the vacuum pump diaphragm, used bake cleaner to clean out the vacuum hoses, and a shop vac to suck oil out the brake booster. I also replaced my rubber brake hoses leading to the calipers. Because of the oil contamination I may in the end need to replace the check valve leading to the brake booster. My vehicle does not have a vacuum modulated trans. so it was/is not affected.

I'll start by checking the Check Valve on the Booster line. how much trouble is replacing the diaphram in the Vac. pump?

Replacing the diaphragm wasn't difficult, a little time consuming. This is an earlier 123 model with a 4 cyl. and the plunger versus the piston type vacuum pump.
On this particular model I took off the cooling fan, 4 bolts (no cluth stuff), loosened the p/s pump and took off its belt to give me easier access.
If your vehicle is a 123 model I have the factory manual that addresses the diaphragm replacement for both type pumps. Also if the vaccum pump is similar or the same on your vehicle to a 123 I could copy and fax over the instructions. The Hayes manual skips over it.
The telltale sign for me that the original diaphragm had seen better decades was when I took the vacuum pump hose off the brake booster and the inside of the hose was wet with oil.

Mccan:

I have similar problems in my Volvo with a faulty, intermittent vacuum pump. Will tear it apart when warm weather comes.
The "Mighty Vac" is worth its weight in gold. Seriously helpful for all kinds of vacuum issues. Cuts the diagnosis time immensely. I love that little deal with all its attachments (they all come with the package 29.95).
Don