[DOUGLAS MARTIN]

My 1980 300D has a vacuum issue. These are the symptoms.
When starting the car, after a half a minute the brakes feel 'normal'. After a short drive to the first stop sign at low speed, the brakes go rock hard - much pedal pressure needed to stop. after a drive of 1/2 mile, the brakes feel normal again during slowdown to 2nd stop sign, then they go rock hard again. If I accelerate quickly to about 45mph, if the brakes are used they remain hard as a rock. If I get on the freeway, after a mile the brakes are fine again if I only touch them once. Two applications and the brakes are hard as rock. I have replaced the diaphragm and cleaned the valves in the vacuum pump. At a stop sign the climate control will stop working if the brakes are applied. If the car is put in park at a light the climate control keeps working. I measured the vacuum at the booster - when the pedal is depressed vacuum drops way down. The booster is leaking, Yes? I'll replace it. But would these same symptoms manifest if the valves in the vacuum pump are worn out?

 

[AMSOIL300CDT]

your vacuum pump and valves sound like they are fine, since you regain vacuum pressure upon release of the brakes. another simple test would be idle the motor, press hard on the brakes, and actuate your doorlocks. if they do not function (as they shouldnt now), thats ok. release the brakes, wait about 5 seconds to allow ur vacuum pump time to build some pressure, and actuate again. if they work again, ur pump is good to go. sounds like ur booster is bad. replace, and u should be fine again. i assume u have inspekted the pressure line to the booster from the motor for leaks. if none then, uv got a bad booster.

 

[benmaples]

What about your check valve or am I barking up the wrong tree?

 

[DOUGLAS MARTIN]

Thanks for the replies. When I depress brake pedal the door locks still work. The check valve and line to air pump from booster is new. Although I have checked entire vacuum system inside and out and have found no leaks, I'm going to plug lines leading into firewall and run car with vacuum line only to booster and transmission bleed valve again. If there are no leaks there then it must be the booster, right? An expensive booster...

 

[DOUGLAS MARTIN]

I blanked of the vacuum lines from the pump to the climate control, res, ignition, etc., and drove the car. Brakes still got rock hard but not so quickly. Then I blanked the vacuum lines to the transmission - I could only drive in 1st gear - brakes got rock hard. Connected things back up and once again I have some boost then rock hard brakes and climate control goes off if waiting at light with foot on brake pedal. I have rebuilt air pump by replacing membrane and cleaning valves. It could be the pump or the booster. Possibly the valves in the air pump? Any sure fire way to tell? I put sealer around joint from mc to booster to check for leaks and nada. Has anyone had these same symptoms? I would like to avoid throwing parts at problem. Also, I had engine valves adjusted - they were tight - and the smoke out of exhaust has nearly vanished.

 

[tomswift]

You said you replaced the diaphram... was it perforated?

Due to past perforations in my diaphram, oil had progressed up into the rubber climate control lines and swollen them until they no longer held vacuum. Check some of the lines visible on the passenger side (remove a panel) and see if oil exposure has gotten under your dash. If yes, you absolutely have to replace EVERY hose (the rubber ones) to get leaks out of the system.

 

[DOUGLAS MARTIN]

I replaced the climate control lines and connectors and replaced / rebuilt the vacuum actuators. The door, trunk, and fuel filler door work without a hitch. Last night the climate control system worked well until I had to wait at a light with foot on brake - as usual climate control system shut down. When I put the trans in park at a light the system keeps working.

Also - there is a vacuum reservoir in the left front fender - it shows up on shop diagrams. How does one access the resevoir to check fittings? I have checks lines for vacuum leaks and found none but would like to see if fittings are in good shape.

 

[tomswift]

The method I would take at this point is to buy/pull out a vacuum pump with gauge ($20-30 at PepBoys) and start isolating system lines at the firewall and pull a vacuum and see what leaks down fast. The kit I bought came with a "T" so you can plug in systems like the locks to the diaphram pump if you don't want to hand pump for 5 min to achieve vacuum.

Try this:
-Unplug everything, block off all holes, check pump vacuum. Should be 21-28", likely at the low end. Turn off engine, vacuum should remain if check valve is okay.
-Check engine shutoff solenoid at rear of injection pump
-Check key shutoff valve (vacuum lines to ignition switch)
-Check locking system (most likely suspect since it is complicated and somewhat hidden)
-Verify check valve is oriented and working properly on transmission vac valve (plastic widget on top of injector pump)
-Verify Vac regulator on transmission is holding vacuum
-Verify idle vac shutoff valve atop the valve cover if equipped
-Since you have a turbo, check any vac systems associated with it
-Check vac to EGR valve if equipped
-And last but certainly not least, check brake booster for vac integrity.

You may need a vacuum diagram to trace the exact lines for your car.. go here: http://www.mercedesdismantlers.com/diesel_vacuum_system

I have been through vacuum hell and returned... you can do it too, and it is not that much trouble if you just take your time and break down the system into small "branches" that serve only one function. If you have questions about the above instructions, leave your email, we will chat.

 

[tomswift]

I just went back and reviewed all your comments about the actions you have taken.... I have a first test I would like for you to run:

Have three or 4 plugs for vacuum lines in your pocket

Unplug the Vacuum modulator atop the fuel injection pump (white or yellowed plastic lumpy widget). You stated that you drove with this disconnected, and it would not get out of first gear, but what truely happens is that the transmission will wind up until it reaches its built in 'redline' and then I promise it will shift (let's do this with the motor warmed up, okay?). YOu may have to take it to the highway to make that 2nd and third shift.

Since you said the brakes worked momentarily after driving for a bit, they should work properly at least for one application now. If this does not cure the problem, plug off the climate control and then the locking system. These are the three likeliest systems to leak.

If these quick tests did not determine a problem, go back to previous response and attack with vac pump in hand. Buena Suerte

 

[DOUGLAS MARTIN]

Thanks for the comments. What is puzzling is the randomness of the brakes. When starting up from cold, SOMETIMES the brakes have boost at low speed, sometimes not. When warm, the brakes SOMETIMES have boost, sometimes not. The only consistent symptom is the climate control shuting off when idling with foot on brake - when idling in park climate control works fine. The door locks work even after a day of sitting. I replaced the vacuum valve atop the cam cover and the trans valve atop the injection pump and don't want to throw parts at problem. It seems the vacuum system evacuates air and everything works except when I apply the brakes and the system seems to 'wind up' creating a vacuum once again and therein lies the rub - I believe it is the booster - my local MB parts man says he is more than happy to sell me a new booster but says the problem may be the pump. I get about 20+lbs. of vacuum from the pump with things blocked off until I depress the brake pedal. Then the vacuum plummets to about 5lbs. Should the pump supply enough power to overcome this drop - is this normal - or is my booster leaking? Or are those pesky little valves in the air pump shot? Or will I never know until I have emptied my wallet for new parts and the mystery remains unsolved?

 

[benmaples]

You have vacuum at your pump & all other vacuum controlled stuff works. I say it's the brake booster. Don't go to a dealer, shop around on the web. How about aoutohausaz right here in the list of advertisers on home page. Check some other sites as well. You should be an auto tech, you have "must know for sure" attitude it takes to keep from wasting peoples money but it looks like you have narrowed it down to me. you could always disconnect booster & drive it like that just make sure you alternate feet so you don't end up with your right calf three times as big as the left[].

 

[DOUGLAS MARTIN]

Tomorrow I am going to perform one last test to make sure the vacuum pump is pumping with enough strength. If that tests AOK I'll go get a booster and be able to diesel around. If not, then the pump gets replaced and the old one gets hammered.

 

[DOUGLAS MARTIN]

RE: Vacuum Mystery / HAUNTED BOOSTER

HAUNTED BOOSTER?

I disconnected the main vacuum line from the booster, trans, and interior, attached a vacuum gauge and started the engine, plugging the booster connection with my thumb. 18 lbs of pressure. Let my thumb off and sealed the end once again. 18 lbs of pressure. Revved(sp?) the engine. 22lbs of pressure. Seems the vacuum pump is short of breath. Will examine check valves and repeat test. Seems like a new pump is called for and would explain why boost goes away. It would seem...so...I plumbed the booster in a similar manner and used a mity vac. Booster held 15 lbs of pressure - until I had a neighbor depress brake pedal - pressure went to ZERO. Booster has a leak? Then with pedal depressed, I actuated mity vac once again. Booster now held pressure. WHAT GIVES?

 

[DOUGLAS MARTIN]

RE: Vacuum Mystery/Mystery Solved

Mystery no longer! I replaced the booster and all systems go go a go go. The back of the old booster was coated in brake fluid but the master cylinder doesn't leak a bit - seems the previous owner replaced the master cylinder but neglected the booster. Damn, what a pain but oh how sweet now that the brakes and climate control work as designed. I purchased a tested used booster from Sun Valley Dismantlers for a fraction of the price of a new one and it came with a warranty to boot. Thanks for your comments.