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Did the brakes but now the pedal still seems soft.

Started by girlwithoutaplanet, 26 July 2014, 06:23 PM

girlwithoutaplanet

1979 300SD


Hi folks,

So a few weeks ago I noticed my brake pedal getting soft on occasion and then the brake pad wear sensor light came on a few times.  So, went ahead and replaced the front brake pads.  Turns out the front right caliper was stuck and had worn the pads down quite a bit (enough to reach the sensor), incidentally, the left side was totally fine.  Replaced front right caliper.  Bled the brakes but the pedal still was too soft.  Replaced the brake master cylinder.  That fixed it!  The pedal had the perfect stiffness to it while bleeding the brakes after doing all that work.

Also, the two rubber nubs on top of the brake master cylinder reservoir were badly cracked and needed replacing.  So I got two new ones the other day and put them on.  While doing that, I noticed how there is a little lever that gets pushed down and up.  Are these what create the pressure within the system?  My old Volvo had what looked like a small diaphram on the cap for the reservoir and you had to push that in to help with the pressure in the system.  The cap on my 300sd does not have this diaphragm I'm used to seeing.

Now, nearly a week after all this work, the pedal seems not as stiff as it should be.  Brakes seem to work just fine.  I'm not losing any fluid.  and all four lines were bled to the point of watching perfectly clean fluid come out and no air bubbles at all.

Is it possible that some air got into that bottom portion of the reservoir while I was changing out the rubber caps?  What exactly is the purpose of those caps and the lever below them? 

Thanks.. I really don't want to try to bleed the brake lines again unless I know it's going to make a difference.  It's the only thing I can think of, though.

Ideas?  Suggestions? 
1979 Gold 300SD - Gogo, The Lady of Metal

ters

Brake (Vacuum) booster needs replacing.

Ters.

13B

Following on from what Ters said, can you leave your foot on the brake after you switch the car off and see if the pedal "hardens up" (so to speak) after about 30 seconds and pushes your foot back with considerable force...  if so, investigate your brake booster.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

girlwithoutaplanet

Ters and 13B,

Thanks for your replies.  I did as suggested and the pedal remains soft.  My understanding of when a brake booster goes bad is the pedal gets more stiff and is not easy to push.  My brake pedal never feels "too" stiff. 

I put my foot on the pedal, start the car, and the pedal slowly pushes down.  Not to the floor, it never goes that far down. 
1979 Gold 300SD - Gogo, The Lady of Metal

ters

Quote from: girlwithoutaplanet on 26 July 2014, 06:23 PM
1979 300SD


Hi folks,

Replaced the brake master cylinder.  That fixed it!  The pedal had the perfect stiffness to it while bleeding the brakes after doing all that work.

Also, the two rubber nubs on top of the brake master cylinder reservoir were badly cracked and needed replacing.

Now, nearly a week after all this work, the pedal seems not as stiff as it should be. 

The master cylinder you replaced was brand new, or used new?  Or you used new master with old reservoir (no problem).... Pedal going soft is an indication of bad booster, OR seals in master cylinder not sealing properly in cylinder bore.

Did you bench-bleed the new master cylinder before instalation?

Ters.

girlwithoutaplanet

Quote from: ters on 29 July 2014, 12:57 AM


The master cylinder you replaced was brand new, or used new?  Or you used new master with old reservoir (no problem).... Pedal going soft is an indication of bad booster, OR seals in master cylinder not sealing properly in cylinder bore.

Did you bench-bleed the new master cylinder before instalation?

Ters.

The master cylinder was brand new, purchased from a shop in town that I trust (I know a guy that works there, have gotten many parts from them).  So yes, new mc with the old reservoir. 

Did not bench-bleed the mc, as I don't have that set up available to me.  So we carefully, and for a very long time, bled each brake line. 

Thanks.
1979 Gold 300SD - Gogo, The Lady of Metal

Jakob

When you replaced the caliper, did you rebuild it?  If so, reading through this thread might be helpful: http://forum.w116.org/mechanicals/6-9-brake-problems-getting-desperate-here/60/

girlwithoutaplanet

Quote from: Jakob on 29 July 2014, 06:40 PM
When you replaced the caliper, did you rebuild it?  If so, reading through this thread might be helpful: http://forum.w116.org/mechanicals/6-9-brake-problems-getting-desperate-here/60/


Jakob,

It was easier for me to just get a brand new caliper.  I actually had trouble getting the springs and pins back in with the pads, so I took it down to the local MB shop and they got it all in for me. 

Somethings else that reminded me about...  There were these round sheet metal covers that went over the rubber seal for the piston (shaped like a life-saver, you know?).  The original caliper did not have them.  I heard the mechanic ask one of their w116 guys if the those covers should be on there and he said yes.  Does that sound familiar to anyone else?

1979 Gold 300SD - Gogo, The Lady of Metal

girlwithoutaplanet

Quote from: Jakob on 29 July 2014, 06:40 PM
When you replaced the caliper, did you rebuild it?  If so, reading through this thread might be helpful: http://forum.w116.org/mechanicals/6-9-brake-problems-getting-desperate-here/60/

The thread you linked is great, btw.  Whether or not it's applicable right now, it is good to know!
1979 Gold 300SD - Gogo, The Lady of Metal

girlwithoutaplanet

As of today, the pedal seems much more stiff.  I'm about 100mi into these new brakes and it does seem like they're getting broken in.

Hopefully just a false alarm and they just needed to be broken in.  If they don't, I'll be back with cries of woe :)
1979 Gold 300SD - Gogo, The Lady of Metal

oversize

Did the rotors have a lip on them or were they machined?  What bed-in procedure did you use?  Those metal inserts over the seals stop the seals from popping off and also help to align the new pads during installation
1979 6.9 #5541 (Red Bull)
1978 6.9 #4248 (Skye)
1979 6.9 #3686 (Moby Dick)
1978 6.9 #1776 (Dora)
1977 450SEL #7010 white -P
1975 450SEL #8414 gold -P

girlwithoutaplanet

Quote from: oversize on 30 July 2014, 09:43 PM
Did the rotors have a lip on them or were they machined?  What bed-in procedure did you use?  Those metal inserts over the seals stop the seals from popping off and also help to align the new pads during installation

Thanks for the info on the metal inserts; good to know they belong ;)

After finishing bleeding the brakes, we took it a few blocks away to a long stretch of road that is mostly uninhabited at night (industrial area).  I did the drive, carefully slow down, drive, carefully slow down thing for about a mile then went down to the nearby highway where I could increase my speed and again carefully slowed down over and over a few times.

Since then, I have mostly done urban driving, rarely exceeding speeds of 40mph (~65kpm) and braking very carefully. 

The rotors were in great condition, actually.  The right side (with the stuck caliper) did have a very minor lip on it.  Instead of having it machined, we put the new pads into the stuck caliper and put it back on the rotor.  I had to drive to work the next day (10mi round trip) and couldn't get a new caliper for a few days.  That stuck caliper with the brand new pads wore down the small lip on the rotor, it actually worked out the way I needed it to. 

I had a pro-diesel mechanic helping and advising. 

I've had the car for just over a year now and have put only a few thousand miles on it.
1979 Gold 300SD - Gogo, The Lady of Metal

oversize

Once you've pumped up the pedal to push the new pads against the flat surface of a new or machined rotor, you can then bleed the brakes.  To bed in new pads drive up to no more than 60kmh, make sure there's no-one behind you and hit the pedal hard to reduce your speed to about 20kmh.  Do not come to a complete stop.  Drive 300m or so before repeating the process about 5 times.

What you're trying to avoid is high temperatures and low pedal pressure that will glaze the surfaces and cause poor performance and squealing.  It's a bit like bedding in new rings
1979 6.9 #5541 (Red Bull)
1978 6.9 #4248 (Skye)
1979 6.9 #3686 (Moby Dick)
1978 6.9 #1776 (Dora)
1977 450SEL #7010 white -P
1975 450SEL #8414 gold -P