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How to bleed master brake cylinder

Started by raueda1, 05 June 2020, 04:45 PM

raueda1

This seems to be a common problem, yet I can't find the process.  Last summer I replaced calipers, rotors, hoses and bled the system - I thought.  Car has been sitting since then while i worked on other problems.  Now all that remains is the brakes.  I tried both the person-on-the-pedal and pressure methods.  I'm not getting any bubbles out of the calipers whatever, and flow seems weak.  The pedal is spongy no matter what.  It's the "stepped tandem main cylinder" with "Teves expansion tank with 1 contact insert" (as described in manual).

There have been other threads mentioning this problem.  The solution seems to be "bleed the master cylinder," but no indication of how.  On some cylinders there seems to be a bleeding nipple, but not on mine.  FWIW, I have no reason tot suspect that the master cylinder is bad.  It worked perfectly when the car was laid up to do the brakes. 

Quick advice appreciated!  The brakes are now all that's keeping me off the road after 10 months.  Thanks and cheers,
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

BCDC

Use gravity. I replaced the brake fluid in mine the other day by cracking all wheel bleed points so that the fluid in the lines is running though and just kept topping up the master cylinder with fluid until all the lines were flushed with the new stuff. No problems.


BCDC
1978 280SE Cream with blue interior. With SLS

BCDC

Just some extra info on procedure of the above method. Hoist car, remove wheels, place buckets under each wheel, then do as the other post says then after it is flushed with new fluid in the master cylinder tighten bleed points then wash everything around the brakes with water to disperse the mess, reinstall wheels. Should be good to go.
1978 280SE Cream with blue interior. With SLS

raueda1

Quote from: BCDC on 05 June 2020, 06:36 PM
Just some extra info on procedure of the above method. Hoist car, remove wheels, place buckets under each wheel, then do as the other post says then after it is flushed with new fluid in the master cylinder tighten bleed points then wash everything around the brakes with water to disperse the mess, reinstall wheels. Should be good to go.
OMG, you gotta be kidding me!  Thanks, I'll try tomorrow.  How long does this take, more or less?
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

BCDC

I let it run though for a half hour or so. They are small lines so give it a bit of time, especially if there is air coming out.
1978 280SE Cream with blue interior. With SLS

raueda1

No luck.  I also tried a pressure bleeder and everything else under the sun. I can only think that the fully operational master cylinder (!!#$@!&€%!!) got wrecked from pumping during bleeding.  Numerous people warn of this problem.  I've got a spare master, will try that.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

rumb

I am not a fan of the 2 person pump and hold method. Perhaps because my wife interprets my instructions differently.  :o

I use a Harbor Freight vacuum bleeder and it has worked well for me numerous times.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

raueda1

Quote from: rumb on 06 June 2020, 04:01 PM
I am not a fan of the 2 person pump and hold method. Perhaps because my wife interprets my instructions differently.  :o

I use a Harbor Freight vacuum bleeder and it has worked well for me numerous times.
That may be the next step.  Today's path was different.  There seem to be a lot of caveats here and elsewhere about ruining a previously good Master Cylinder by pressing the pedal all the way down to bleed.  Which, of course, is exactly what I did (it always worked fine for me in the past).  So I started down the failed MC path.  Rather than get a kit or rebuilt MC I disassembled, cleaned and inspected a spare MC.  It looked very clean inside - no corrosion, scoring, rust etc.  Seals all looked perfect too, not distorted, flattened etc.  So I put back together and will give it a shot.  If that fails the vacuum bleeder will be next.

It's making me nuts!  This is all that stands between sitting on a lift and driving the damn thing!
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

I use a "one man bleeder", pretty simple just with some hose and an empty plastic bottle.
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

Case closed.  The solution is so stupid and embarrassing that I wouldn't post except for pointing out a disturbing trend I've noticed recently. 

First the solution:  rear calipers were switched left/right so bleeders were on the bottom.  Why?  They bolted right up this way and connected directly to the short hydraulic line spanning the rear of the axle assembly.  I figured that this was the right way and assumed that there was some kind of internal change to the caliper making this OK.  False!  Orienting then with bleeder on top required new (and quite different) hydraulic lines.  Thank god I had the stuff on hand to make them.  But I did, and it bled perfectly. 

The trend:  These were supposed be exact , compatible, drop-in replacements.  Superficially they were except that the hyrauling fitting was located differently.  It was a "these aren't identical but can be made to work" scenario.  They came from eBay after a search of other suppliers failed (Pelican Parts, Autohaus AZ and others).  It's like there's a shortage of rear calipers and the eBay guys are willing to do the "these aren't identical but can be made to work" thing.  I had a very similar experience with rear accumulators.  What I got on eBay would physically fit but were just wrong.  So I'm done with eBay.  My attitude is now that if the something isn't identical original then it's wrong and goes back immediately.  At least the car now has a nice, reconditioned master cylinder.   Caveat emptor.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

TJ 450

Good work getting that sorted. Where is the fluid line fitting located on these different calipers, in the middle? If so, they are definitely wrong for the car.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

raueda1

Quote from: TJ 450 on 07 June 2020, 08:50 PM
Good work getting that sorted. Where is the fluid line fitting located on these different calipers, in the middle? If so, they are definitely wrong for the car.

Tim
This is the wrong caliper made to work.  Note bleeder on top where it should be.  Also note where the brake line goes (angled down slightly).  The fitting should be where the arrow is, pointed slightly up.  As it is, a new line was needed to make it work.  It's very very close to the rubber suspension buffer, so I need to double check that there isn't interference.  And to repeat - all this because rear calipers are seemingly in short supply and so somebody decided that close enough is good enough.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

Dave I recall going through the same shenanigans 5 years ago when I redid my brakes.  Part# for rear 1234200683/1234200583.  First ones I sourced were wrong as well, and had to return them to the vendor.  I can imagine that now 5 years later it will be even harder to find the right ones.
Daniel
1977 450 SEL 6.9 - Astralsilber

Randys01

AAAh.   !!   "the old Upside down caliper bomb in the bon bon box trick Chief." :-[
It takes me back 40 years.  340  E55 Chrysler Charger. Those rotten  calipers looked so right on the wrong side upside down......overhauled every iota of that crappy brake system coz it would not bleed.  When it was finally sorted, this tub had the best brakes in town.!

raueda1

Quote from: Randys01 on 10 June 2020, 03:51 AM
AAAh.   !!   "the old Upside down caliper bomb in the bon bon box trick Chief." :-[
It takes me back 40 years.  340  E55 Chrysler Charger. Those rotten  calipers looked so right on the wrong side upside down......overhauled every iota of that crappy brake system coz it would not bleed.  When it was finally sorted, this tub had the best brakes in town.!
Hahaha, seems like a lot of people have done something similar.  2 friends (old corvettes) did something similar, though not on their 'vettes.  One of them ended up having his car towed to the dealer!  The damn thing of it is, in this case it was done cause these calipers were the wrong ones.  So hmmmm, install with fitting in the right place but upside down?  Install right side up but wrong fitting?  We are all being tested by mysterious forces we can't see, feel, taste or smell.
-Dave
Now:  1976 6.9 Euro, 2015 GL550
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0