vacuum pump has 22hg reading @ idle still have hard braking? 1980 300sd - PeachParts Mercedes-Benz Forum

musikpal · #1 03-16-2013, 02:05 PM

My 80 300sd get about 3 times of braking before the pedal becomes hard.

After about 5 seconds @ idle the power came back and again gone after 3 times of braking.

Eliminating components one by one, the problem came down to either the vacuum pump, the brake booster, the master cylinder seal (I didn't think about this one until now) or, a very small chance, the big tube between them (small chance because it read 22 Hg @ idle).

I was very convinced that the brake booster was the culprit because of the 22Hg reading so I did one last test before I changed the brake booster.

I disconnected all vacuum lines and plugged them, except for the one that I connected to a vacuum gauge.

I then put the car on an inclined drive way, put it in neutral with emergency brake off and hold my feet against the brake pedal the vacuum dropped below15Hg and finally went to almost 0 Hg if I wouldn't let go.

The hardness of the pedal was directly related to the reading.
Apparently too much vacuum was lost when I applied brake.
I thought I must have lost vacuum through one of the valves inside the brake booster so I replaced it with a rebuilt booster by Cardone.
However, the problem is still there.

So the question here is:
Can a vacuum pump that read 22hg @ idle still be the cause of hard braking?

In another word, Does a vacuum pump that has 22Hg reading @ idle pass the test and is 100% a good vacuum pump?

Or I just happened to hit a jackpot by receiving a bad new (rebuilt) unit?

I forgot to mention also the brake(so must be the vacuum) may seem slightly better when it's warm than when it's cold.

.

Diesel911 · #2 03-16-2013, 10:04 PM

I don't entirely follow your tests.

Hook your Vacuum Gauge to the little Nipple on the Main Vacuum Line that is closest to the Vacuum Pump. If you have 2 little Nipples on you Vacuum Liner remove the Hose from that and put a cap over the Nipple to seal it. The remove the Main Line from the Power Brake Booster and Plug off the Vacuum Line.

The above cuts off the Vacuum from the Brake Booster and all the other components.

Start the Engine and see what Vacuum you get; you will be reading the Vacuum from the Vacuum Pump only.
If that is low remove the Check Valve that screws into the Body of the Vacuum Pump and look in sided to see if the parts are still there and in good condition.
If they look OK spray out the insides with WD-40 to clean it out; install it and retest.

If the Vacuum Pump test out OK find some way to connect the Mighty Vac to your Brake Booster and see if it will hold a vacuum.
If it is not holding Vacuum change the Master Cylinder Seal/O-ring and also look down inside of the Brake Booster to see if there is Brake Fluid inside of it.

bolzano · #3 03-17-2013, 01:21 AM

Quote:

My 80 300sd get about 3 times of braking before the pedal becomes hard.

After about 5 seconds @ idle the power came back and again gone after 3 times of braking.

Eliminating components one by one, the problem came down to either the vacuum pump, the brake booster, the master cylinder seal (I didn't think about this one until now) or, a very small chance, the big tube between them (small chance because it read 22 Hg @ idle).

I was very convinced that the brake booster was the culprit because of the 22Hg reading so I did one last test before I changed the brake booster.

I disconnected all vacuum lines and plugged them, except for the one that I connected to a vacuum gauge.

I then put the car on an inclined drive way, put it in neutral with emergency brake off and hold my feet against the brake pedal the vacuum dropped below15Hg and finally went to almost 0 Hg if I wouldn't let go.

The hardness of the pedal was directly related to the reading.
Apparently too much vacuum was lost when I applied brake.
I thought I must have lost vacuum through one of the valves inside the brake booster so I replaced it with a rebuilt booster by Cardone.
However, the problem is still there.

So the question here is:
Can a vacuum pump that read 22hg @ idle still be the cause of hard braking?

In another word, Does a vacuum pump that has 22Hg reading @ idle pass the test and is 100% a good vacuum pump?

Or I just happened to hit a jackpot by receiving a bad new (rebuilt) unit?

I forgot to mention also the brake(so must be the vacuum) may seem slightly better when it's warm than when it's cold.

1985 300SD. Have done exactly the same tests as you did with the same findings. Why? Hard brake pedal. Pumped brakes on and off quickly to flash the brake lights on and off to warn the driver behind me because I was slowing drastically for a turn with a young careless driver in a Honda riding my bumper. Found pedal was hard after pumping brakes x5. Later realized hard pedal meant ran out of vacuum but braking distance was normal if I pressed hard.

Changed master cylinder before I knew how MB vacuum works. New master cyl to brake booster seal. No change. Changed to junkyard supposedly good brake booster. No change. Changed to Cardone rebuilt booster. Maybe a minimally worse than original or junkyard booster. Changed vac pump check valve. No change. Installed new vacuum pump. A little improved.

Test drove someone else's older W126 300SD. Ran out of power brake assist after pumping brakes 3 times from 25 mph coming to one stop. Same as mine.

Concluded that running out of brake assist might be normal. Or if not normal, there might be nothing I could do to fix the issue. I avoid pumping the brakes and get prepared to use 2 feet on the brakes if needed. Not needed lately since I no longer pump the brakes. If I brake, I push the pedal down and hold as needed.

musikpal · #4 03-18-2013, 04:52 PM

Quote:

Originally Posted by bolzano

1985 300SD. Have done exactly the same tests as you did with the same findings. Why? Hard brake pedal. Pumped brakes on and off quickly to flash the brake lights on and off to warn the driver behind me because I was slowing drastically for a turn with a young careless driver in a Honda riding my bumper. Found pedal was hard after pumping brakes x5. Later realized hard pedal meant ran out of vacuum but braking distance was normal if I pressed hard.

Changed master cylinder before I knew how MB vacuum works. New master cyl to brake booster seal. No change. Changed to junkyard supposedly good brake booster. No change. Changed to Cardone rebuilt booster. Maybe a minimally worse than original or junkyard booster. Changed vac pump check valve. No change. Installed new vacuum pump. A little improved.

Test drove someone else's older W126 300SD. Ran out of power brake assist after pumping brakes 3 times from 25 mph coming to one stop. Same as mine.

Concluded that running out of brake assist might be normal. Or if not normal, there might be nothing I could do to fix the issue. I avoid pumping the brakes and get prepared to use 2 feet on the brakes if needed. Not needed lately since I no longer pump the brakes. If I brake, I push the pedal down and hold as needed.

In your case, you might want to check the the pump seal, I have read about the thickness of new seal,that came with new pump, being the cause of the failure.
Vacuum pump lesson learned

bolzano · #5 03-18-2013, 11:06 PM

Quote:

By bolzano. Changed master cylinder before I knew how MB vacuum works. New master cyl to brake booster seal. No change. Changed to junkyard supposedly good brake booster. No change. Changed to Cardone rebuilt booster. Maybe a minimally worse than original or junkyard booster. Changed back to junkyard supposedly good brake booster.
Changed vac pump check valve. No change. Installed new vacuum pump. A little improved.

Quote:

Originally Posted by musikpal

In your case, you might want to check the the pump seal, I have read about the thickness of new seal,that came with new pump, being the cause of the failure.
Vacuum pump lesson learned

Installed a new Pierburg vacuum pump. Brake boost a little better than old vacuum pump. Vacuum measures a little stronger. Still runs out of brake boost if I pump the brakes x 3. So I think it is not l likely that the seal thickness is the problem.

musikpal · #6 04-25-2013, 03:52 AM

Quote:

Originally Posted by bolzano

Installed a new Pierburg vacuum pump. Brake boost a little better than old vacuum pump. Vacuum measures a little stronger. Still runs out of brake boost if I pump the brakes x 3. So I think it is not l likely that the seal thickness is the problem.

Changed to Cardone rebuilt booster. Maybe a minimally worse than original

We can all now conclude that Cardone boosters are junk. I returned the remanufactured Cardone and put a used booster(came off of 85 w126) n my problem is 90% fixed(after about 6 consecutive brakes the pedal becomes a little hard, but vacuum is regained after a few seconds).

pgringo · #7 04-25-2013, 10:08 AM

i have added a vacuum canister from a volvo 240 into the leaf_catcher/fire_trough in my w126 to get a little more time worth of vacuum. i have plans for a 12v setup i will document and share once it's installed.

Phillytwotank · #8 03-17-2013, 10:52 AM

Seems like all the symptoms of a bad brake booster.

Diesel911 · #9 03-17-2013, 11:43 AM

Symptoms of a bad Brake Booster?

While it could be the Booster what makes it seem like that is not the case is that He gets 3 Brake Pedal Pumps before He gets hard Braking.
To Me that indicates that the Check Valve is closed and Vacuum is being trapped between the Booster and the check Valve and he is getting the 3 Brake Pedal Pumps just as He is supposed to if the Vacuum Pump failed completely.

Then there was My own experience with the same issue. Had, vacuum about the same has he did and would get a hard Brake Pedal after several applications of the Brake. In My case cleaning out the Check Valve with WD-40 cured that over 4 years ago and I have had no issue since that time.

bolzano · #10 03-17-2013, 03:47 PM

Quote:

Symptoms of a bad Brake Booster? While it could be the Booster what makes it seem like that is not the case is that He gets 3 Brake Pedal Pumps before He gets hard Braking.
To Me that indicates that the Check Valve is closed and Vacuum is being trapped between the Booster and the check Valve and he is getting the 3 Brake Pedal Pumps just as He is supposed to if the Vacuum Pump failed completely.

Good info. Which check valve is this? The one on the vacuum pump like circled in red in the first picture? Or the one on the line to the brake booster like the second picture. And in the upper right corner of the third picture diagram?

Quote:

Then there was My own experience with the same issue. Had, vacuum about the same has he did and would get a hard Brake Pedal after several applications of the Brake. In My case cleaning out the Check Valve with WD-40 cured that over 4 years ago and I have had no issue since that time.

Just spraying the check valve with WD40 fixed your problem? Sounds good.

Diesel911 · #11 03-17-2013, 09:11 PM

Quote:

Originally Posted by bolzano

Good info. Which check valve is this? The one on the vacuum pump like circled in red in the first picture? Or the one on the line to the brake booster like the second picture. And in the upper right corner of the third picture diagram? Just spraying the check valve with WD40 fixed your problem? Sounds good.

The one that screws into the Vacuum Pump in the Red Circle in the first Pic.
Others have not been so lucky with the Vacuum Check Valve and have unscrewed it and found the parts inside are Broken or even entirely gone.

Note; if the Plastic Check Valve in the Middle of the Main Vacuum Line was not working the Brake Pedal would not have that 3 Brake Pedal depressions before you got the Hard Pedal.

bolzano · #12 03-18-2013, 12:48 AM

Quote:

Originally Posted by Diesel911

The one that screws into the Vacuum Pump in the Red Circle in the first Pic.
Note; if the Plastic Check Valve in the Middle of the Main Vacuum Line was not working the Brake Pedal would not have that 3 Brake Pedal depressions before you got the Hard Pedal.

Running out of brake assist can still happen in my 1985 300 SD. Old check valve had no internal parts remaining. First picture. Unfortunately, changed to a new check valve (2nd and 3rd pictures) on the old vacuum pump with no improvement. Old check valve had no internal parts remaining. Another new check valve came on the new vacuum pump. Power brake assist improved but could be could still run out of power brake assist by pumping the brake pedal. Tested brake booster. Holds vacuum from a vacuum pump. Car has some brake assist after sitting several hours. What is there remaining to test or fix?

Ether · #13 03-18-2013, 12:09 AM

Can a vacuum pump that reads 22 hg at idle still be bad? Yes it can.

I have the same vehicle and experienced the same symptoms you are describing. My problem turned out to be the vacuum pump even though it showed good with the vac gauge directly on the output. After replacing the check valve, the main line from pump to booster and confirming the booster did not leak, i finally replaced my vac pump with a used unit and my problem was solved. I surmise that the original vac pump simply couldn't supply enough vacuum under load.

Easy enough to check the brake booster for leaks though it does take a bit of effort with a mity vac.

musikpal · #14 03-18-2013, 07:23 AM

Thank you all, what a good info you guys provided. Thank you Ether, that's what I really need to know, that I can't eliminate the vacuum pump even if it shows 22Hg.