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Discussion Starter #1 (Edited)
Hello! I drive a 1979 300D with about 332,000 miles. Last week I was experiencing a soft traveling brake pedal near floor and at times the brake pedal would become stiff and kind of sensitive to the touch (not hard to push down like if vacuum pump was bad). My mechanic looked it over and told me that the master cylinder was no good and appeared to be leaking brake fluid into the booster upon further inspection.
Also, my brake pads looked unevenly worn on the drivers side (as if caliper had been sticking), so I went ahead and replaced both front calipers with rebuilt ATE's, turned both front rotors, installed brand new semi-metallic pads, and bled both front and rear wheels. Now I noticed that the brake pedal gets stiff especially during traffic in morning and afternoon drives, but after an hour or so pedal feels like it frees up and becomes soft again. Could this be a symptom of a bad brake booster, vacuum pump (and we did check vacuum at line near booster prior to removing master cylinder and appeared to be fine) or some other brake vacuum leak that I am not aware of? How can I test booster to see if it is good?
Thanks, thanks!!:confused:
 

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Hello! I drive a 1979 300D with about 332,000 miles. Last week I was experiencing a soft traveling brake pedal near floor and at times the brake pedal would become stiff and kind of sensitive to the touch (not hard to push down like if vacuum pump was bad). My mechanic looked it over and told me that the master cylinder was no good and appeared to be leaking brake fluid into the booster upon further inspection.
Also, my brake pads looked unevenly worn on the drivers side (as if caliper had been sticking), so I went ahead and replaced both front calipers with rebuilt ATE's, turned both front rotors, installed brand new semi-metallic pads, and bled both front and rear wheels. Now I noticed that the brake pedal gets stiff especially during traffic in morning and afternoon drives, but after an hour or so pedal feels like it frees up and becomes soft again. Could this be a symptom of a bad brake booster, vacuum pump (and we did check vacuum at line near booster prior to removing master cylinder and appeared to be fine) or some other brake vacuum leak that I am not aware of? How can I test booster to see if it is good?
Thanks, thanks!!:confused:
It does have the symptoms of a bad Master Cylinder and you do have a leak which makes it kind of conclusive.

Be sure to mop up the liquid brake fluid from inside of the Power Brake Booster as the Fluid deteriorates Rubber. Some would say the Brake Booster is ruined by the Brake Fluid in it but most People ignore that.
So that is your call.

Sticking Calipers is a common problem.

There is little valves and Seals inside of the Power Brake Booster that can go bad but that seldom happens. But, those can also mimic a Vacuum Problem.

A good place to start would be to check your Vacuum. When fully working it is supposed to be 22-24 Inches of Mercury. When I was having intermittent trouble it was around 17 Inches of Mercury at the lowest and 22 when it worked OK.
In my case it turned out the Check Valve on the Vacuum Pump was full of Gunk. I sprayed it out with WD-40 and it is still working 5 Years later. Don't use Brake Cleaner as it some times makes Plastic Parts Sticky.

The Check Valve is the Fitting that Screws into the Vacuum Pump and the Main Vacuum Line screws onto it. That is not to be confused with the other Check Valve that is the Plastic One near the Brake Booster.

When you trouble shoot try to deal with one item at a time and then test. That way you will know what actually fixed the problem.
 

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Could also be a restricted rubber brake hose to a caliper. Because of age, I would replace at a minimum, the 2 front caliper hoses. They flake off internally and cause restricted fluid flow back to the master when you release the pedal. Frequent brake application in city traffic would speed up the symptom.
 

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It is important to remove the fluid from the booster - the FSM says you should really replace it if it gets contaminated with brake fluid. (I can understand why people would rather try and get it back to life though not a cheap part to replace)
 

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I had a similar issue. I would start with the brake hoses. $8 each.. cheapest thing to fix first in my opinion. If they look swollen, this is definitely your problem.
Does your booster look like there was fluid dripping on the front of it? Boosters are expensivish.
 

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I had a similar issue. I would start with the brake hoses. $8 each.. cheapest thing to fix first in my opinion. If they look swollen, this is definitely your problem.
Does your booster look like there was fluid dripping on the front of it? Boosters are expensivish.
There is an O-ring at the end of the Master Cylinder where it butts up against the Power Brake Booster.

That O-ring is supposed to keep the Vacuum in the Power Brake Booster and that also means if there is a Brake Fluid leak from the Rear of the Master Cylinder no Brake Fluid should leak past that O-ring to the outside.
 

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Discussion Starter #7
Brake Issues Solved!

Thanks so much for all your quick replies! Finally fixed the brake issues on my '79 300D this past weekend which turned out to be the brake booster. Was able to get a re-manufactured ATE from a local indy shop for $135.00. Brakes feel very nice, thanks again!
 

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Since day 1 of my ownership I've noticed that the bottom part of the brake booster had signs of fluid leakage. I can barely read any markings off of the booster so this is probably an old unit.

How long do the brake boosters usually last?

Thanks so much for all your quick replies! Finally fixed the brake issues on my '79 300D this past weekend which turned out to be the brake booster. Was able to get a re-manufactured ATE from a local indy shop for $135.00. Brakes feel very nice, thanks again!
That's a very good price. Found mine not holding vacuum (slow leak), so I opted to find a new unit. :eek:

None of the usual online parts stores had them available (back order, out of stock, etc). Lucked out from an Amazon seller and got a new TRW unit.
 

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Since day 1 of my ownership I've noticed that the bottom part of the brake booster had signs of fluid leakage. I can barely read any markings off of the booster so this is probably an old unit.

How long do the brake boosters usually last?


...
I think the number one cause for booster failure is fluid leakage so I guess the question is more related to how long will seals in a master cylinder last. This I guess is about 15 to 20 years depending on loads of factors (that are too tedious to list!)
 

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Don't mean to hijack this thread, but am having similar problems.
I am having intermittent booster failures.
I have replaced both front calipers, the master brake cylinder and the booster. Once in a while (like yesterday in heavy highway traffic) my booster will fail and I get a very hard pedal. I almost got to see what the underside if a semi trailer looks like. Anyway I have checked vacuum and am getting over 20", think like 22". Every time the brakes get hard they will start working after a few minutes. Hope someone can point me in the right direction of what to look for.
When it first started I found a bad trunk lock actuator and plugged those lines. Tightened up booster as it seemed a little loose. I put a round o- ring between master cylinder and booster that seems to seal better.
Like I say it is an intermittent problem that may only happen once a week, but when it does it can get exciting.
Thanks for all the help you guys give.
Rod
 

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When was the last time that you changed your brake hoses coming off the calipers? Maybe there is some internal swelling going on. I had a similar thing happen. Brakes would get hot, pedal would get high and hard, then it would be nice and normal again. Almost like there was some sort of pressure buildup. When I changed the hoses, things got better.
 

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Don't mean to hijack this thread, but am having similar problems.
I am having intermittent booster failures.
I have replaced both front calipers, the master brake cylinder and the booster. Once in a while (like yesterday in heavy highway traffic) my booster will fail and I get a very hard pedal. I almost got to see what the underside if a semi trailer looks like. Anyway I have checked vacuum and am getting over 20", think like 22". Every time the brakes get hard they will start working after a few minutes. Hope someone can point me in the right direction of what to look for.
When it first started I found a bad trunk lock actuator and plugged those lines. Tightened up booster as it seemed a little loose. I put a round o- ring between master cylinder and booster that seems to seal better.
Like I say it is an intermittent problem that may only happen once a week, but when it does it can get exciting.
Thanks for all the help you guys give.
Rod
With lots of new components (such as the booster) and what seems to be a healthy pump - but that heavy use / frequent use causes the problem - I can only conclude that you have a leak somewhere. Sorry to say it but systematic testing of each section of the system is required. It is tedious but a methodical check should find the culprit.
 

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I think the number one cause for booster failure is fluid leakage so I guess the question is more related to how long will seals in a master cylinder last. This I guess is about 15 to 20 years depending on loads of factors (that are too tedious to list!)
Well, I confirmed that the booster is leaking vacuum, Friday night.

Did the same test but this time I kept pumping for a few minutes. Still nothing on the gauge, and if it does reach anything above "zero" it gradually goes down to the needle rest.

New TRW brake booster en route, due to arrive Thursday. I searched AutohausAZ ($290, backorder), PelicanParts ($290, out of stock), MercedesSource ($350), then finally Amazon ($300).

New MB unit is $600! :eek:
 

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Well, I confirmed that the booster is leaking vacuum, Friday night.

Did the same test but this time I kept pumping for a few minutes. Still nothing on the gauge, and if it does reach anything above "zero" it gradually goes down to the needle rest.

New TRW brake booster en route, due to arrive Thursday. I searched AutohausAZ ($290, backorder), PelicanParts ($290, out of stock), MercedesSource ($350), then finally Amazon ($300).

New MB unit is $600! :eek:
So that's how the brake pedal should feel with a proper working brake booster! I've been driving all along with a stiff brake pedal. But now I realized there was another vacuum leak - traced it down to the trunk lock vacuum element (original was dated 1982). All others in the doors are new, replaced a while back.

Yes $600 for a new MB booster sucks!! :(

But my $300 TRW unit sucks just as well for less :)
 
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