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.