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300SD Brake Booster bad?

9K views 55 replies 8 participants last post by  jzchen 
#1 ·
Ok, I know that there are already a ton of threads on this, but after searching and reading them, I still need some clarification.

I just got done completely overhauling the suspension myself, and I really mean completely! Got new tires put on, and then got an alignment. Got to drive it for 3 days (the best 3 days ever). Then, when I left work, I hit the brakes and they were rock hard. I could still stop, but obviously the brake assist was out. Got home, pulled into my garage and my car wont turn off. Checked the vacuum line to the shut off valve right away and it was still connected..my heart sank :frown Its always something!

I knew right away it was porbably something wrong with the vacuum system so I started reading through the posts and started eliminating what I could....Here;s what I have done so far

-Checked brake fluid level, right where is should be
-Started the car, hooked up the mityvac to the shut off valve, pumped it twice and the car turned off, so the shut off valve is good
-Inspected all the vacuum lines and fittings, they are all good as I've replaced most of them in the engine area
-I then tested vacuum coming directly from the pump and it was reading right at 22hg
-Disconnected vacuum line at the brake booster, removed and capped all vacuum lines by the check valve, and the was vacuum still reading at 22hg, so no leaks in the line
-Completely removed the large vacuum line and blew air into the line, I can only blow air one way through the line so I assume that means the check valve is working
-I then reconnected everything but kept all the lines by the check valve capped except one that was hooked up to the mityvac. The vacuum started building VERY slowly, I waited about 10 mins and it climbed up to about 15hg. Went and hit my brake pedal, it was back to normal on the first press, but after the that, it got super stiff, looked at my mityvac and it was at 5hg at climbing very slowly
-I then disconnected the vacuum line again at the booster, hooked up my mitvac directly to the brake booster and pumped away. It took a while to pull vacuum on it, but I got it up to about 10hg with the mityvac and then watched the vacuum go down to 0.
-Lastly, just to be sure I checked underneath the dash and made sure the lines were still connected to valve on the back of the ignition tumbler, and they were

So my questions are...

Does this indicate a bad booster? It seems like, from what I've read, that usually the brake pedal gets soft when the brake booster goes out or is going bad.
Is there a better way to check the booster?
Am I right in assuming the check valve is good if I can only blow air one way through it?
Is there anything else I should check

Thanks for the help!
 
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#2 ·
I have replaced the master cylinder, brake booster, large vacuum line, and vacuum pump in my W123, but it has been a long while so forgive me if I am not accurate.

Brake booster starts leaking, visually under the hood, and pedal goes to floor when it fails.

I think what you describe is master cylinder failure, the stiff pedal....




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#4 ·
Thanks for the responses

Brake booster starts leaking, visually under the hood, and pedal goes to floor when it fails.

I think what you describe is master cylinder failure, the stiff pedal....
I don't think its the master cylinder because of the car not turning off as well, that points me towards a vacuum problem. The brakes still actuate, just have to push really hard

When a master cyl. fails it gives you a soft sinking pedal and usually a red brake warning light. When a booster fails, it gives you a rock hard pedal that is hard to depress. Remember what the booster does. It provides boost or assist to lessen braking effort. In most cars you can pull the hose off the booster and you get a rush of trapped air because the booster stores vacuum for the next brake application. . Based on your post, I think you need a booster.
Ok, though each time I disconnected the vacuum line from the booster, there is a small rush of air but not very much. Could it be bad but still hold some amount of vacuum?

Also, is a proper test of the plastic check valve to simply disconnect the line at the booster and then blow in it, if air goes in its bad? Do I need to cap any of the small lines?
 
#3 ·
When a master cyl. fails it gives you a soft sinking pedal and usually a red brake warning light. When a booster fails, it gives you a rock hard pedal that is hard to depress. Remember what the booster does. It provides boost or assist to lessen braking effort. In most cars you can pull the hose off the booster and you get a rush of trapped air because the booster stores vacuum for the next brake application. . Based on your post, I think you need a booster.
 
#7 · (Edited)
When a booster fails, it gives you a rock hard pedal that is hard to depress. Remember what the booster does. It provides boost or assist to lessen braking effort. In most cars you can pull the hose off the booster and you get a rush of trapped air because the booster stores vacuum for the next brake application. . Based on your post, I think you need a booster.
Car nut is correct, replace the brake booster and make sure your vac. lines and connectors do not have leaks and are all intact. As long as all that is ok and the vacuum pump is good you should have no problems accepting this.
To test it reconnect everything back up as was normal with car off pump up the brakes until the are hard keep pedal depressed and start the engine, if booster is ok the pedal should go softer. If booster is bad the pedal will stay hard even with engine running.
Cheers.
 
#5 ·
The check valve is a one way valve. It allows vacuum into the booster and holds it. Can you blow through it one way and not the other? If its holding a small amount of vacuum I would recheck the specs on the vacuum pump. Is there a way you can use a MityVac to pump in vacuum to the booster and verifies it holds?
 
#6 ·
Yes I blew into the check valve and air would only travel one direction. I also did pump vacuum in the brake booster with a mityvac. It took a while to get it up to pressure but then as soon as I stopped pumping, it would fall back to zero fairly quickly.

I have to keep driving it as its my daily driver, and yesterday the brakes came back and the car turned off all day. Today it was working as well until I actually pulled into work, then brakes went hard again.

Does this point to a possible blockage in the lines? Or possibly the o-ring between master cylinder and brake booster? MercedesSource says thats a common leak point as well.

Thanks
 
#8 ·
Ok, thanks I will do that. I picked up a booster at the pick-a-part this weekend. Is there any way to test the "new" booster off of the car?

Also I had read that its possible to pull the booster without removing the master cylinder and bleeding the brakes. Is this true? Because it seemed impossible when I pulled the booster in the junkyard. The booster needed to come forward quite a ways before I could get it out.
 
#9 ·
I changed the booster and master cylinder at the same time. I bought mine from AutoZone with lifetime warranties in 2011. I don't remember having to have to bleed the brakes. The metal lines still had brake fluid near the top when I removed the master cylinder. Try carefully and you may be surprised you don't need to like what happened to me.

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#10 ·
On some cars you can remove the master cyl. from the booster and pull it forward (lines attached) it depends if the push rod comes out of the booster or not. In my experience, If you must remove the master cyl. from the lines, simply reinstall the lines tight and pump the pedal and crack open the lines at the master cyl.
 
#13 ·
Awesome, that sounds like a lot less work. When I got the booster at the pick-a-part, I got the master cylinder as well because it looked brand new. It must have been recently replaced. I will just plan on replacing it knowing that I don't have to do a complete bleed procedure if I remove it.

Another thing to consider, after all is said and done, you need to be sure that the system is properly bled. I thought that I had a bad brake booster, and it turned out that I had some leaking calipers, and air in the system. Once I replaced the bad calipers, and bled it the best that we could, the pedal was still very hard. It took my indy (who KNOWS how to properly bleed the system) bleeding it again before the pedal was as it should be. It is not rocket science, but MUST be done properly if you want the system to function as it should!

Rob
I have checked all the lines and calipers and there are no leaks. I had my calipers and lines replaced 2 years ago so they "should"/better be. Also I definitely think its more vacuum related as the problem has been intermittent. As well as the car not turning off when the brakes are hard. But I will definitely bleed the system if the problem persists after replacing the master cylinder and booster.

Does anybody know if there is a way to test a booster with out it hooked up to the car? I would like to test the used one before I go through all of the work to replace it just to find out its garbage too. I can't really afford a brand new one right now because I just got done redoing all of the suspension....

Thanks
 
#12 ·
Another thing to consider, after all is said and done, you need to be sure that the system is properly bled. I thought that I had a bad brake booster, and it turned out that I had some leaking calipers, and air in the system. Once I replaced the bad calipers, and bled it the best that we could, the pedal was still very hard. It took my indy (who KNOWS how to properly bleed the system) bleeding it again before the pedal was as it should be. It is not rocket science, but MUST be done properly if you want the system to function as it should!

Rob
 
#14 ·
Hi there JZ,

Air you could check the vacuum feed line to the brake booster with your MityVac that might be a good idea.. I'm with you ou as to the condition being a vacuum leak as both the engine won't shut off aaaaaand brake pedal effort becomes astronomical!

I wouldn't think the booster to have an intermittent problem generally speaking.. In my experience they usually are good.. or when bad.. they are bad.. in fact I've yet to experience one 'waffling on the fence' as you describe..

A weak / dirty check valve or cracked vacuum feed can give the exact same result I believe....

I have changed out my booster vacuum line 2X in thirteen years on the SEC.. The first time it developed a crack and brakes lost power assist as you describe.. This last summer I had it disconnected and twisted it.. loosening the line fit at the check valve.. Apparently, it had gotten brittle over time & all those hot/cold heat soaks hadn't helped I guess.. Dam! I had to order a replacement... :/

In my eyes, that big line is the most fragile part in the chain.. and subject to vibrations & movement... The booster, on the other hand, is just kinda quietly sittin' there waiting for something to happen.. :)

Swapping out a bad line is waaay easier than the booster as you have noted...

Good luck man,

MBL
 
#16 ·
Yes, repair manual instructions note this, but I did not know how to bench bleed it, so I just put it in. It couldn't be worse than the pedal going to the floor with almost no braking so I just tried, and if needed/spongy, I would take it carefully two miles to the dealer. For some reason I believe/remember it worked perfectly fine....


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#18 ·
I don't seem to have a receipt for the brake bleed. (My receipt folder is probably 2 inches thick.)

You could step on the brakes to see without moving the car as well.....

I've twice had the symptom of pedal to the floor with practically no braking on the W123. First time we just drove it to the dealer. Second time I did what I did. I'm not going to lie and say we had it towed because it was unsafe....

I now currently have a pressure brake bleeder, by Branick, but I did not back then......

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#19 ·
If you do invest in a pressure brake bleeder, be sure the reservoir is not wobbly/loose, or try to tie it down to the master cylinder. I found out the hard way that if it is loose AND you don't tie it down, on my W201, that you can pop it off. It cost me a lot to go to the local dealer and pay retail for the two gaskets, then go have the engine bay washed because brake fluid "exploded" out, which I had to sign a waiver that anything happen during the washing is not the car wash's fault...

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#20 · (Edited)
OK, I'm a little late to this party, but... My 2p:

First, if you connect the mity vac directly to the booster and it doesn't hold vacuum, the booster is bad. Period. And, that jives with your other symptoms: low vacuum in the whole system, ok vacuum with the booster disconnected, and failure to shut off the engine. The only time it should not hold vacuum is while you are releasing the pedal after applying the brakes.

Second, if you open the brake system at the master cylinder you must bleed the system afterwards. Period. There is simply no way to replace the master without introducing air into the system unless you submerge the whole car in a vat of brake fluid. (Not recommended.). Now, the fastest way to bleed it is to push the air back out through the master. You can sometimes do this by pushing the pistons in the calipers back in, or you can do it by forcing fluid in through the bleeders with a pressure bleeder, just make sure you don't introduce a bubble into the caliper in the process. (There will be air in the bleeder itself and you have to get it out and not force it back into the system.) Best to do a complete bleed job, though, and get all new fluid into the system.
 
#21 · (Edited)
To all who say you need to bleed first:. I decided to try first.

What happens if you get a leak in a line or a caliper? Does the master cylinder just not allow fluid to flow through? Why do pressure bleeders work? Because the fluid readily flows downwards. The system is designed so that if you spring a leak in a caliper that fluid will be allowed down so braking is still available, that a low brake fluid situation develops, and a light on the dash warns you to stop...

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#26 ·
update, but still confused

So it finally thawed out here in Idaho. After having hard a$$ brakes and manually turning off my car all winter, I finally braved the cold garage and got to work on this. I replaced the brake booster, the master cylinder seal, and flushed/bled the brakes. Unfortunately, it didn't fix anything. After losing my mind for a few minutes, I started going back through the whole system. The one thing I never did was actually pull out the external check valve on the vacuum pump. I had just briefly looked in there when I tested the output but not actually removed it. When I pulled it out, a few pieces fell out of it. Finally!!! I had found the problem!! So, I took out the broken pieces and put the bad valve back onto my pump until I ordered and received the new one. The next morning, I pull out to go to work and my brakes are there. I pumped the brakes a bunch and they didn't get hard. I immediately turn the keys to see if the car will turn off and it does. Its been 3 days now and everything is still working properly. How in the heck is this possible??? If the broken check valve was the problem in the first place, how is everything now working after taking all the internals out of the check valve, making it useless?
 
#28 ·
Well now I'm really losing it. I just went to lunch in my car after my previous post and all of the problems are back. I'm totally lost on what his could be now if all the check valve does is hold in vacuum while the car is off. How can be perfectly fine for 3 days and then go back?
 
#32 ·
I have verified steady pull of about 21 from the vacuum pump. I never used the booster I pulled from the gas 380 at the pick-a-part. I bought a reconditioned one because I didn't want to go through the hassle of putting in a booster that could go out in a year.
Unfortunately in an old car you can start looking at the other older parts that might have just failed. In my W123 I think I changed the main vacuum line, the vacuum pump, the master cylinder and the booster around the same time.....
This is where I'm at now, I'm think I'm going to just get a new main vacuum line and check valve, if it persists after that, then I'll get a new pump

Does anybody have the part number for the external check valve on the vacuum pump? The part number I had wasn't correct.....

Thanks for all the help everyone
 
#31 · (Edited)
If you have no boost and it won't shut off automatically you either have a bad pump or a big leak. I'm guessing messing with the check valve disturbed things just enough to bring the pump back to life for it's last gasp. Only other possibility I can think of is there's some smegma in the lines that is blocking things.
 
#33 ·
I'm having trouble finding the correct part # for the main vacuum line with the built in check valve. All the ones I'm finding for my 1982 300sd are coming up with part# 1264301229 (1st picture). This hose shows 2 ports closer to the pump end of the line and my hose has the ports right next to the check valve so that they sit behind the firewall by the fusebox (2nd picture) . Is this just the updated part and will require putting new lengths of vacuum line in?

I think I found the part # for the external check valve on the vacuum pump. Can anybody verify that 0049971872 is correct?
 

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