hard pressed brakes - PeachParts Mercedes-Benz Forum

michaeld · #1 12-11-2005, 12:37 AM

I've got a 77 450 sel w/ 125k that has a clear issue w/ the brakes. The car stops, but it takes a very firm foot on the pedal before the brakes engage. Also, I hear the faint sound of escaping air pressure as I step on the pedal. I have a VERY tiny leak in my master cylinder, but it has been so small I haven't worried about it. Could a leak in a master cylinder be causing my symptoms? Ordinarily, I would attribute the "ffsssshhh" when I step on the brakes to a leaking brake booster, but I there is no mention of a brake booster in the Chilton manual. Is the master cylinder inegrated w/ the booster in this model? I would obviously like to fix my brakes before I bump into someone the hard way, but I don't want to replace the wrong part. Suggestions?

Also, anyone w/ the know-how to answer the above question can probabely answer an unrelated one regarding fuses for the same model: is there a good upgrade for the ceramic fuses? I have heard that corrosion is a problem w/ these ceramics.

#2 12-11-2005, 12:43 AM

Do you have something that looks like this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41OK01GN2V&year=1977&make=MB&model=450-SEL-003&category=All&part=Brake+Booster

michaeld · #3 12-11-2005, 02:23 AM

Thanks for your reply, craig.
yeah, i have one of those thingies. I'm a lot embarrased that I looked in a manual instead of taking a good solid look under my hood. I have a power brake booster, all right. Mine IS leaking fluid - I can see it leaking from where it joins the master cylinder. But it is a tiny trace leak, as I check my brake fluid routinely and have had to add very little.
I find I have to step hard on the brakes, but the car actually does brake pretty well. As I step on the brakes, I'm starting to hear the faint sound of escaping air pressure in the driver's compartment, a sort of "ffffffsssssshhhhffff sound. I'm trying to decide if I need to replace the master cylinder, the brake booster, just a seal, or even just bleed my brakes. what do you think

420 SE · #4 12-11-2005, 02:53 AM

Start with a brake bleed, it's cheap and easy. If you do have a leak, albeit smal, it may be drawing air into the system.

Fuses - it's not the ceramic fuses which cause the corrosion but the aluminium strip. If you have ceramic with copper there is no problem. If you do have ceramic with aluminium - change them ASAP for ceramic/copper. You will have to look around for them - they are getting harder to find (over here anyway) In fact I haven't been able to find 8A copper for a while so I take them out every so often and clean the connections.

Cheers

Ferdman · #5 12-11-2005, 05:57 AM

michaeld, you can order brass and copper alloy replacement fuses from **************.com

Ara T. · #6 12-11-2005, 06:29 AM

I experienced this with my W123 leaky master cylinder. Just change it. Look for a rebuild kit if you don't want to spring for the $100 or so... changing the MC resulted in a huge increase in stopping power. I can just lightly rest my foot on the brakes when I see a red light and the car will stop no problem. And if you really jam on the brakes, well be careful cause they'll lock up when they never would before. And this is with a brake booster that isn't putting out full brake assist.

I do agree that you should bleed and flush the brake system, it will help for a while at least. I found that my pedal got harder after I bled but it still took a good amount of foot pressure to stop, especially when going down a hill. Just didn't inspire much confidence.

t walgamuth · #7 12-11-2005, 10:33 AM

best to change the mc right away. it wont heal itself. it will only get worse. you dont want it to fail. it will of coures fail when you need it the most, when you are pushing hard on the brakes. this is no time to economize. a rebuild kit will work if you can find one. the new mc are cheap enough that i no longer rebuild them at least on benzes. after changing the leaky mc you will be able to tell if the booster is ok. before that you are spinning you r wheels.

a bad booster cannot cause brake fluid to leak.

tom w

michaeld · #8 12-12-2005, 02:03 AM

Thanks a whole bunch for your feedback, guys. Let me process it back to make sure I get the gist.
While it is apparently my brake booster that is creating the 'vacuum leak' escaping-pressure sound, and might also be making it more difficult to apply stopping pressure on the brakes, I should actually be much more worried about the master cylinder.
I do see brake fluid in trace amounts, primarily on the brake booster housing at the M.C. juncture. I understand that this is due to a leaking master cylinder, rather than a booster, because boosters don't leak brake fluid.

I would MUCH rather replace my master cylinder than my booster. The thought of crawling under my dash trying to reach 4 inaccessible bolts just makes me .... shudder.

Here's my question: let's suppose that I follow what sounds like good sense and replace my master cylinder, would that be adequate to fix my brake integrity issue if I didn't also replace the booster? Does a bad booster just make one have to apply more foot pressure, or does it actually contribute to the risk of a braking system failure?

My 'leadfoot' is more than adequate to apply pressure to stop the brakes w/o the assist of the booster, but I definitely don't want to find myself unable to stop. Would a master cylinder replacement and system bleed be adequate?

P.S. Thanks for the info on the fuses. I will try to get the copper-type.

t walgamuth · #9 12-12-2005, 04:47 AM

description i am not able to be sure of the condition of the booster. it may be ok. so the mc which is leaking is obviously bad needs changing so you need to do that to adequately evaluate the booster. changing the booster at the same time will save a little time, perhaps a half hour, but it is expensive maybe 200 or so and why change it if it is not bad? they usually last a very long time.

tom w

michaeld · #10 12-13-2005, 12:39 AM

It sounds like I ought to replace my master cylinder, and then check out my brake system. This sounds like a good plan for someone who is budget-minded but still wants to remain safe. Thank you all very much.