The wife's '82 300TD Wagon (166k) keeps getting a case of hardbrakepedalitis. To my wife, this is the same as brake failure. To some guys, this wouldn't matter but I actually like the old girl. I did some investigating and this is what I found. I disconnected the main vacuum line at the pump and attached a vacuum gauge. I get 18". This is marginal. I reconnected the main vacuum line and attached the gauge at brake booster end. Again read 18". I checked the check valve in the main vacuum line and it works correctly. Also, main vac line is new. Disconnected the cabin vac line and installed gauge there. First push on the brakes worked fine but dropped vacuum to about 15". 3 or 4 more pushes on the brake pedal and vacuum is zero. After this, vacuum builds back to 18" in about 1 minute.

Do you think it's a bad booster or is the pump not moving enough air (or not making enough vacuum) to regenerate what is lost by the booster.

Thanks.

wes

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

It's not uncommon for the vac pumps to wear out and the need for a new one.

It does sound like you have a failing vac pump!

Disconnect the vacuum line from the pump to the brake booster and plug the brake booster end. Make sure the line holds vacuum and that you have a gauge in the line. Confirm the reading on the gauge. If it still reads 18" (it may read higher), then unplug the line. The gauge will go to zero. Plug the line again, and measure the time, in seconds, that it takes for the gauge to build back to above 18".

Report back if the gauge reads higher than 18" when the line is plugged.

Report back on the time it takes to reach 18" after you have unplugged and replugged the line.

Better do as he says!

I was gonna recommend the same but but but, actually, I don't know the set up on an iron 5, hay I like that phrase!

What is the vacuum on a 603? I can't remember, but I thought it was higher than 18"...........???

Thanks Brian and BusyBenz.

Did some more testing. I didn't have the ability to test only the main hose. Not using a MityVac. I did connect my vac gauge to the booster end of the main hose with the engine running. Read 18" again. Turned off motor and waited 10 minutes. No drop in vac was noted. With motor running, disconnected gauge so vac was zero. Reconnected and the gauge hits 18" in about 2 seconds. Did the brake pedal pump test again. By pumping the brake pedal I get the vac down to about 4". It takes about 45 seconds to ramp up to 18" vac again.

Thanks for helping me.

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

One question regarding this test:

Was the main hose attached to the booster, with the gauge attached to a T in the line? Or was the the main hose disconnected from the booster with the gauge attached to the end of the hose?

The test you are asking about, the gauge was connected to the booster end so it was not connected to the booster. The second test, I reconnected the hose to the booster and T'd in the vac gauge.

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

Which was the second test?

I'm curious to determine if you had the hose connected to the booster, with the gauge T'd into the line, and then shut off the engine and monitored the gauge.

Was this the test where the vacuum held for 10 minutes?

Sorry for the confusion.

The first test was with the main vac line attached to the pump at one end, then the vac gauge was attched to the booster end. When shutoff, the vacuum held for 10 minutes.

The other test I did had the main vac line attached to the pump and the booster. I disconnected the T to the interior and attached my gauge there.

I will do the test having the line attached to the pump and the booster and shutting the motor off. Will let you know what the result is in 20 minutes.

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

OK Brian I did the test you wanted.

The main vac line was attached to the pump, the opposite end was attached to the booster. I removed the line to the car's interior and attached my gauge there.

I ran the car and built up 18" of vacuum. I then shutoff the engine using the manual stop lever. The vacuum immediately dropped to 11". I let it stand for 10 minutes and the vacuum dropped to 9". My feeling is that the leakage can be attributed to the booster.

Thanks for your time.

wes.........

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

Thanks for making that test.

After analysis of all the data, I'm reasonably convinced that the vacuum pump is not working properly.

My conclusions are based upon the following:

1) The pump only achieves 18" vacuum. This is well below the minimum 20" that would be expected. A new pump should be up around 23" or so.

2) It takes far too long to evacuate the booster. If we presume that the booster has no leak, then 45 seconds is way too long to build vacuum inside the booster.

3) The final test shows that the booster will hold 11 inches of vacuum for an extended period of time. If the booster had any kind of leak, no matter how slight, the vacuum would go to zero within ten or fifteen seconds. It did not do this. I cannot readily account for the instantaneous drop from 18" to 11", however, it may simply be the relaxation of the diaphragm. I do not believe the diaphragm is compromised. The drop from 11" to 9" over the course of 10 minutes is not significant. It could simply be a slight leak at the seal where the m/c actuation rod passes through. This small leakage cannot account for the 45 second time to build vacuum within the booster.

So, I would recommend that you get the kit to rebuild the vacuum pump, which is demanded in any case because of the 18" reading. I'm fairly certain that it will cure the problem with inadequate performance from the booster.

Hi Brian,

Thank you for your expert troubleshooting skills and detailed analysis.

Is the rebuild kit the best solution or is it easier to purchase a new pump?

wes............

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

Naturally, it's easier to purchase a new pump. But, it's costly.

To rebuild your existing pump is not too difficult, although I have not done it personally, and there are some tricks to achieving success.

You can do a search on "vacuum pump" and read up on the issues involved.

You can then make a decision if your mechanical skills are up to the task of doing the rebuild.

If you start the rebuild and get stuck, I'm quite sure that there are other members who can talk you through it. Plan on more than one day...........just in case.

It appears that removing the pump is relatively straightforward. Any pitfalls that you know of? If I can get the pump out easily, rebuilding it shouldn't be a problem. I'm a Mech. Eng. and regularily take stuff apart just to see how it works.

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'82 300 TD Wagon (China Blue)
'99 Ram 2500 Cummins Turbo Diesel

Me too, It's the putting it back together part that always gets me in trouble.