BRAKE troubleshooting - PeachParts Mercedes-Benz Forum

rickmay · #1 05-13-2011, 04:46 PM

Just got a brand, spanking new 1983 240D, and I am the mechanic in training.

When I first picked up the car, the brakes had a fairly hard pedal, more evident at low speeds (still the case). I have tested the vacuum pump and it exceeds Mercedes specs. And while I have some slow seepage in my vacuum system from the doors and the reservoir, it is so minimal that it shouldn't effect braking. The Mercedes test for the pump is to put the vacuum gauge on one of the lines coming off the main line, and leaving the other(s) wide open. With a 10 inch booster, the manual says it should reach 14.6 "Hg in 12-13 seconds. It does it in 6-8 seconds. With the lines all on, I get 21 "Hg, so vacuum looks good.

Anyone know where you get the black rubber vacuum connector that leads into the yellow check valve? The connector looks like a CACTUS. I pushed the feed into the cactus further, closer to where it divides, and that helped the brakes somewhat. I have searched "connector" at a number of websites, and I haven't seen one.

The car sat for 3-4 years, so the brake pads and rotors may be quite dirty, enough so, that they contribute to this somewhat. The mechanic that services my other cars swears by Akebono pads. He "never has had a problem with them." Interestingly, they have ONLY a 12 month, 12,000 mile warranty, so has anyone used them? Has anyone heard how well they wear? I figure if I clean the rotors and install new pads, this will help. The guy that sold me the car liked putting silicone on at least everything rubber, so it wouldn't surprise me if his "help" helpfully sprayed some on the rotors, as in, near the tires.

The fluid definitely needs changing as it is brownish, and not clear. I could see where this can cause a hard pedal, as it could be 2-4 years old.

The pedal also drops about 1/2 inch to 1" once I have come to a stop and leave my foot on the it, but I don't seem to be losing fluid. Is the drop in the pedal a sign of the master starting to fail? If I push once or twice it brings the pedal up again.

I see no fluid in and around the master, its connections, the plastic reservoir or the exterior of the booster.

I would like to pull the booster and check it out. I plan on replacing the "O" ring between the master and booster, but since I am not losing fluid, the booster should be dry. What do you look for once the booster is pulled? so can you see inside, shake it, what? What else do I look for?

I have held off on changing the brake fluid to check the master, or should I start with this?

If anyone wants to chime in on any or all of these questions, it would be greatly appreciated. I will be in the garage trying to fix the very quite horn.

BrassMonkey · #2 05-20-2011, 03:18 PM

Lots of stuff here.

Lets start with the easy first. You have to pump the pedal or it sinks 1" after you stop. If you don't have a leak its probably not the MC (could be - but probably not). Lets go step by step.

1) is there enough fluid in the MC res? - As pads wear the calipers are extended further and there is less fluid in the res - this can cause you to need to pump the brakes.
2) Wear - See 1
3) Tiny amount of air in the lines - air compresses - hydrolic fluid doesn't. When you step on the brake and there is air it will get squeezed in and the pedal will sink. This could be anywere - perform a good bleed and if it was air - this will fix it (assuming there isn't a leak).

So - go ahead and do the flush - bleeding from the farthest caliper and once clear new fluid is coming out of the bleeder move to the next farthest etc.

If that doesn't fix it - then start looking at the MC - these are fairly inexpensive - but you dont want to replace if you dont have to. Usually when the MC fails though - there will be a leak because its the fluid pushing past the rubber seals that causes you to lose brake pressure.

OK - the vacuum - You are on the right track and its good you know the VP works. Go ahead and repalce the lines from the check valve to the booster.

I'll need a pic of the cactus to know exactly what you are referring to. But if you have a firm pedal - you need more vac to the booster.

Does the pedal get better at higer speeds? not sure why that would be -

vstech · #3 05-20-2011, 10:51 PM

in addition to vacuum problems, a hard pedal can be caused by seized calipers, and bad flex hoses...

rickmay · #4 05-21-2011, 10:01 AM

VSTECH

According to Mercedes manuals, I am getting enough vacuum, and the vacuum comes up more quickly than the specs. The leaking to the reservoir and doors is quite small that it should not affect braking. If I pump the vacuum to 20-21 "Hg, it would take a half hour to an hour or more to drop down to 0," so vacuum to the booster does not seem like it should be a problem. The booster, also, seems to be holding vacuum. I let the car run for 3 minutes, and let it sit there for 5-6 minutes. I disconnected the main line to the booster, and there was a big woosh of air. My callipers are at 80%-90% and working well. Even the parking brake performs like it is a new car.....which is rare. How can a flex line cause a hard pedal? None of my rubber hoses, at the callipers, are cracked, and they are not leaking.

BrassMonkey
The pedal seems more hard at slow speeds. Once over 5 MPH, the pedal seems fairly normal. Even though, the pedal might be slightly hard at 5 MPH+, I think that could be from the fact that the brake fluid is old, and may have taken on water while in storage (I read somewhere, that over time, the fluid can take on water). I do not have to pump the brakes when stoppiing, but the pedal does sink the half inch to an inch once I stop and still have pressure applied. Put in other words, the pedal does not sink at all until I come to an almost complete stop, or a complete stop. And even then, it only sinks an inch or less. This last one second drop in the pedal is mainly what I am trying to figure out.

1. MC full with no apparent sign of leaking. Not losing fluid either.
2. Little wear.
3. Air is possible

I will try bleeding the system as soon as I get a new vacuum tester/brake bleeder. Mine walked of from my garage when I was in the house. Some kid must have liked the shiny red and brass, gun-looking toy.

As far as a picture of the CACTUS, here is a photo found online. Different car, but vacuum connector and yelllow check valve is just like mine. 1. the gray line from main vacuum line feeding booster, 2. line feeding ignition, 3. to check valve, to reservoir and to doors, 4. haven't figured where this goes, yet. One rubber line in and three out..... zoom in (open file and hit control ++++++++) I also highlighted its shape in the second photo to clarify shape. WHERE TO BUY ONE?

vstech · #5 05-21-2011, 10:06 AM

brake hoses often deteriorate from the inside. often creating a one way valve inside. and they can often cause the pedal to be very hard to press.
if you don't know how old the hoses are, change them. the booster may be holding vacuum, but have a ruptured disc inside... I'm just tossing out possible issues you might have.
have you tested each and every piston on your calipers to be certain they are not siezed? debris can also cause hard pedal, as can a bad MC, although the more common MC failure is no pedal... or at least a falling pedal. get a pressure bleeder, or make one, then flush each and every caliper, and see if anything changes.

Yak · #6 05-21-2011, 11:17 AM

The fourth, green, line goes to your climate control. For new parts try the buy parts link or call. You probably want the X-connector (it's not a cactus). Possible p/n 1170780045

If you want to troubleshoot the brake vacuum completely, you could block off this line and test your brakes. You'd lose the functions on the line: shutdown, central locking, climate control. But unless there's been some significant mods, it shouldn't affect brakes.

The lotus forum has an excellent cutaway of a brake MC and description of the "falling pedal" that could be caused by small internal leaks in the MC. I don't know if the MB MC is the same or similar enough, but the engineering seems plausible.

http://www.lotustalk.com/forums/f101/brake-pedal-sinks-after-while-21866/

Check the grounds for corrosion for your "quite" horn. There are also carbon brushes in the steering column stalk switch, or the contacts themselves behind the horn pad. Or a bad horn...

rickmay · #7 05-22-2011, 05:03 PM

Good info, YAK

Thanks, and thank you John.

Do you know if the Brown vacuum line goes to the ignition switch, and then to the shut-off valve?

Rick

rickmay · #8 05-22-2011, 06:20 PM

I did the test shown at the link for the booster where you pump the brakes several times, push down on the brake pedal and hold, and then start the car.

I pushed the pedal 5-6 times and then pushed and held. The pedal slowly dropped almost 2 inches. This seems to be the master cylinder, and not air in the system. If it was air it would not have taken the 5-6 seconds to drop the 2". It may have dropped a little more when I started the car, but not a lot.

I don't think the booster is ruptured, as the brakes are not too bad with any amount of speed.

I am going to bleed the brakes anyway, and check the calipers again, but
I looked at my calendar, and it may be a month or so before I check in again.