Brake Booster, master cylinder or Vacuum Pump? 300sd - PeachParts Mercedes-Benz Forum

cousindave76 · #1 10-03-2006, 11:42 AM

I'm sorry but this will be a long post. However, as you know, the symptoms of these lovely, eloquent MB automobiles can not be simply describe in just a few words.

I have an '83 300sd with booster problems. I'm trying to figure out if it's the break booster going bad or the vacuum pump. Or possibly the master cylinder!

Here's what it does... When I press the brake pedal down, it initially feels like it has plenty of stopping power but, when I try to press any harder it's like pressing it against a brick wall. She'll do no more than the initial amount I pressed... i.e. if I'm approaching a stop sign and start applying the brake I have two choices... 1. wait until I'm almost there and hit the brakes hard and it stops fast and hard or 2. press the brake with plenty of time to spare and roll up to the stop sign nice and slow while pressing very hard on the pedal to get it to come to a complete stop.
I've also noticed while sitting at a stop light that is on a slight down hill(causing me to apply extra brake power) that my climate system fades away. The center vent vac modulator does not have enough vac left to stay open.

So that's what it's doing... Here is what I've done to diagnose the cause.

I really don't know a way to check the brake booster itself so I'm hoping that eliminating or identifying the problem from other components in the system with narrow this down.

I've bled the brakes exhaustively... I promise there is no air in the break lines.
I felt no improvement in my brakeing after doing this.

I checked vacuum. I don't have a way to check the vacuum directly off the pump due to fittings I don't have to adapt to my vac gauge. From the main/brake booster vac line I disconnected the vac line that goes to the shift modulator and components. I hooked my vac gauge up at this point and started the car. The vac slowly but eventually climbed up to 23" hg. A good reading I thought but I'm not sure how fast it should get to that reading. Then I pressed the brake pedal down and noticed it sucked all my vacuum down to about 5" hg and sometimes less(I assume that's why I lose climate control while sitting at a stop light). When I release the brake pedal the vac slowly climbs back up to 22-23" hg. ?????

is this my booster, MC or vacuum pump?

Any ideas? Any other testing I should or could do? Your input please!

This is my only car... I would love to be able to stop when needed... for my safety and mostly the safety of others... please help me figure this out.

david s poole · #2 10-03-2006, 04:35 PM

based on your description i would try one more thing.with your vac gauge hooked as you stated dissconnect the vac line from the brake booster and hold finger over booster line end.now see if vac builds much quicker[almost instantly]if not then replace vac pump.

cousindave76 · #3 10-03-2006, 04:40 PM

I love it when I get smacked in the face with logic! Thank's David, I will give that a try this afternoon and post the result of the test here.

Wish me luck... "oh god oh god please let it not be my booster!"

cousindave76 · #4 10-03-2006, 06:24 PM

Ok mr. Poole... I tried what you said and I'm scared of my results.

With the Booster hooked up... when I start the car the vac reading goes from 0" to 22" hg in about 10 seconds.
With the brake booster disconected I start the car... it runs at about 5" hg until I put my finger over the main line. Once I plug the booster connection with my finger it climbs to 22" hg in 2 to 3 seconds...

I'm think my vacuum pump my be good... DAMN! easy fix!
Any input as to the results of my test will be appreciated. Does my vac reading sound normal or should it be an instant jump to 22"hg off the pump?

Now for the fun part. If it's not vac pump... is it brake booster or Master cylinder?

I'm steering away from MC because my problem is somewhat intermitant. I think if my MC went out or is going bad it would just be bad bad and worse. Not sometime ok most of the time bad. Does that make sence?

If the brake booster is faulty, will it or can it suck vacuum from the rest of the system? Or is it normal for the booster to work this way?
Can a brake booster sometimes work ok and then other time not work hardly at all as it is going out/bad?

I hate to just start replacing parts. Get's expensive that way.

As always, I'm very appreciative of help. Thanks in advance!

benzforlife · #5 10-03-2006, 06:50 PM

UNBELIEVABLE, how much these cars are alike. Over the summer, i was expieriencing the same problems, i would hit the brake once, and lets say i was going to apply more pressure, i would then push it again and it would be like 'pushing a brick wall' . I went like this for about a week thinking that it was my pads, for my brake pad light was on, I finally took it in, told my mechanic that i think i needed my pads changed, ok, he says, one of his shop hands jumps into the car to take it around back, and into the shop, in the course of doing so, he almost runns ointo the wall!!!!!!!! mechanic says it could be ur vacum, later rules that its the master cylinder, brake booster, and the break pads, all at the same time,Unbelievabel!!!!!!!!!!!!!

unfortunately it sounds like the same scenario for ureself!!!

cousindave76 · #6 10-03-2006, 07:17 PM

Quote:

Originally Posted by benzforlife

UNBELIEVABLE, how much these cars are alike.

UHG!!! I don't want to hear that! No... I refuse! Ok, there was my tantrum.

Now, on to what you were saying. I think you could be right... it is possible that my booster and MC are both in trouble. I do however have new pads and rotors all the way around. So I know that is not contributing to this problem.

Benzforlife... for wallets sake I hope you are wrong. I will continue to diagnose and find the one TRUE culprit in my case.

whunter · #7 10-03-2006, 07:51 PM

#1. seperate the master cylinder from the power booster, inspect the O ring for damage Picture #A.

#2. Dismount both REAR calipers, squeeze the pistons back in, install a block of wood (with zip ties) filling roughly 2/3 of the caliper pad gap in each caliper, (enough to allow each pistons to come out five millimeters, and impact the wood), stroke the brake pedal (normal force) several times, and inspect for pistons that are siezed, replace BOTH REAR calipers if either is bad. Note: Replace the rubber flex hoses with the calipers.

#3. Dismount both FRONT calipers, squeeze the pistons back in, install a block of wood (with zip ties) filling roughly 2/3 of the caliper pad gap in each caliper, (enough to allow each piston to come out five millimeters, and impact the wood), stroke the brake pedal (normal force) several times, and inspect for pistons that are siezed, replace BOTH FRONT calipers if either is bad. Note: Replace the rubber flex hoses with the calipers.

whunter · #8 10-03-2006, 08:20 PM

Some links that may help you.

I have tried everything... vac booster
http://www.peachparts.com/shopforum/showthread.php?p=997851

Disappearing Brake Fluid
http://www.peachparts.com/shopforum/showthread.php?p=1033187

Brake:
http://www.peachparts.com/shopforum/showthread.php?t=142411

Have a great day.

cousindave76 · #9 10-04-2006, 11:02 AM

wHunter thank you so much for the links and guidance. I had no idea the connection between the MC and booster was critical for holding vacuum. I thought that vacuum was all internal to just the booster. I will separate the two today and let you know my findings.

This very well could be my problem because it fit's my symptoms closely. And if this is my problem, I suspect the added pressure pushing on the MC when I apply brake causes the o-ring between the MC and booster to further lose it's seal and thus causes me to lose the rest of my vacuum. It makes so much sense now.

I know my MC is not leaking into the booster because, except for bleeding the brakes, in the 3 1/2 years I've owned this car I have never had to add a drop of brake fluid to the reservoir.

When I replaced my rotors and pads about 3 months ago... I checked the pistons for free movement and they all seemed okay. I did not however perform the scientific approach you suggested. If I find a good seal between the MC and booster I will perform the caliper check as you instructed.
Thank you for the info whunter... hopefully I will find a bad seal.

I'm actually excited again to go work on this problem!

david s poole · #10 10-04-2006, 02:02 PM

what usually happens here over time is that the master leaks some fluid into booster and this fluid then rots the diaphram.it's possible that po replaced master and didn't tighten nuts,used old o ring,didn't see old o ring and now there are two,knew the master was leaking but didn't bother to suck out the fluid before replacement.as you can see lots of poss scenarios here good luck.

cousindave76 · #11 10-04-2006, 02:36 PM

Quote:

Originally Posted by david s poole

what usually happens here over time is that the master leaks some fluid into booster and this fluid then rots the diaphram.it's possible that po replaced master and didn't tighten nuts,used old o ring,didn't see old o ring and now there are two,knew the master was leaking but didn't bother to suck out the fluid before replacement.as you can see lots of poss scenarios here good luck.

Indeed! I guess I'll find out when I open her up today. I'll be sure to check the booster for any signs of previous leakage or damage.

Thanks Poole

benzforlife · #12 10-04-2006, 08:04 PM

Well, im diffinetly glad to hear that ur breaks didn't give u shear hell like mine!!

cousindave76 · #13 10-05-2006, 11:24 AM

Ok folk's here it is...

My brake booster is bad. Totally sucks! Here is how I came to that conclusion...

I started looking for a new O-ring to go between the master cylinder and the brake booster. I could not find one online so I called my trusty local MB parts supplier. I told him what I wanted and he asked "Why would you want that? It's undesirable to have a No Leak seal between the two because if the MC ever fails it will just fill up the booster with fluid and cause it to go bad too."

Stunned by his comment I asked him "but if the seal leaks between the two won’t that cause a vacuum leak, and subsequently brake loss?"

He then said... get ready for this... "There should be NO vacuum on the master cylinder side of the booster. If there is, that means the diaphragm of the booster is torn."

He then said to try this... "Remove the MC from the booster and start the car. If the booster is sucking vacuum where the MC bolts up, that's a bad booster."

So folk's. That's what I did. I found that with my MC removed from the booster caused a TOTAL vacuum loss throughout the car. The only reason my brakes work at all right now is because my diaphragm has only a SLOW leak. With the MC bolted in place it builds up vacuum and I get one shot at stopping. After that I have to release the brake pedal and wait a few seconds for vac to build up before I can hit them again.

This makes total sense to me. I've never known any booster for any car, by design, to have vacuum on the MC side of the booster... as suggested in earlier posts of this thread. I talked to a booster rebuilder and he confirmed my findings and said that it is without a doubt a torn diaphragm. There should only be vacuum on the brake pedal side of the diaphragm. I told him how I hooked a vacuum gauge up and I get 23” hg until I hit the brake and it drops to 5”hg. He said that it is normal for the vacuum to drop like that when you press the pedal because the booster demands more air volume to be used. He said that the vacuum should however quickly regenerate up to 23” hg as I hold the pedal down. The fact that mine stays at 5” hg proves the diaphragm is torn, which does not allow the vacuum to build up again until I release the brake pedal.

If any of you guys think I'm wrong please tell me... it won't hurt my pride. I'm not an expert here, I'm just trying to apply logic to the problem.

whunter · #14 10-05-2006, 12:18 PM

This is a Mercedes Benz, not a Chevorlet...

The booster functions in a different manner...

There is vacuum between the master and booster...

The rubber O ring is mandatory for proper functioning brakes...

cousindave76 · #15 10-05-2006, 02:36 PM

Quote:

Originally Posted by whunter

This is a Mercedes Benz, not a Chevorlet...

The booster functions in a different manner...

There is vacuum between the master and booster...

The rubber O ring is mandatory for proper functioning brakes...

Wow... I'm stumped. whunter, you've been doing this a while so I'll trust what you say. Question, when I unbolted the master from the booster and pulled it loose, it sucked in air as though it was holding vacuum just fine. The engine had run about 15/20 minutes prior to me removing it. Does that mean the seal between them is, or was, okay? It's a cheep o-ring so I will replace it as soon as I can find one.

So I'm to the frustrated point now. I have a vacuum pump that put’s out 23" hg, a master cylinder that does not leak a drop of fluid. I could not see any signs of fluid or oil buildup inside the booster when I removed the master. I have freshly bled all brake lines starting on the longest to shortest line. All pistons on all calipers are moving when brake pedal is pressed. New pads new rotors all the way around. And yet I have a brake pedal harder than hell to push.

Benzforlife, looks like I'll have more trouble with this than I hoped.

Could this be as simple as the check valve on the main brake line? How do I know if that is still working as it should? And if it's not, would it create a hard pedal problem?

I'm going to go look in a mirror now and tell myself that someday it will be okay... someday.

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode