Brake Bleed Nightmare - 300SD

[James L]

My 1984 300SD (non abs) had the famous brake symptoms; sometimes a soft pedal, push down harder as usual for braking, pump pedal a few times and then all would be ok. Most of the time pedal/braking was ok.
So after reading on this forum, bought/installed a new master cylinder (Meyle).

I can not get a firm pedal (pump and pump and pedal does not firm up at all). I can feel these catch glitches in pedal when pumping.

Things I have tried;

bench bleed master cyliner (twice)
master cylinder o-ring seal to booster good.

assistant pumps pedals as in FSM, no fluid to rear wheels again, no pedal to the floor when bleeder screw released on rear wheels.

So then try my power bleeder, went thru 3 quarts of brake fluid to all four wheels, got all air out, solid fluid. Then I try pedal and it returns as soft as before. Have to really push down to get braking. Again assistant tried to pump after this and can not get fluid to either rear wheels, did get fluid to the two front wheels.

Then tried mity vac and got a little fluid out of rear wheels then it stops and can not get any air or fluid out.

Vacumn at booster is 26-30 inches, when removing vacumn booster hose/pipe I hear a hiss. No oil in booster. Swaped booster hose/valve with my parts car no change.

Rear brake hoses have been changed two years ago.

I have done alot of reading and searching on this forum and I am at my wits end!!! Do I have a bad brake booster? bad new master cylinder? should I reinstall old master cylinder? should I swap parts car booster out? Air in system still? I wish I would have never touched these barkes! I have been at this for a week, alternating between power bleeder, mity vac, assistant pump, gravity bleed. Still can not get a firm pedal (it never even seems to build up when pumping!)

help!

[Junkman]

If the master pumped fluid when bench bleeding, you know that it is moving fluid toward the rear calipers. Perhaps pull a rear caliper line and confirm that you have fluid there. If not, move to the next junction forward. If yes, suspect the caliper. No pedal to the floor when the bleeder is open points to an obstruction. Now you have to find it.

[whunter]

Originally Posted by James L

My 1984 300SD (non abs) had the famous brake symptoms; sometimes a soft pedal, push down harder as usual for braking, pump pedal a few times and then all would be ok. Most of the time pedal/braking was ok.
So after reading on this forum, bought/installed a new master cylinder (Meyle).

I can not get a firm pedal (pump and pump and pedal does not firm up at all). I can feel these catch glitches in pedal when pumping.

Things I have tried;

bench bleed master cyliner (twice)
master cylinder o-ring seal to booster good.

assistant pumps pedals as in FSM, no fluid to rear wheels again, no pedal to the floor when bleeder screw released on rear wheels.

So then try my power bleeder, went thru 3 quarts of brake fluid to all four wheels, got all air out, solid fluid. Then I try pedal and it returns as soft as before. Have to really push down to get braking. Again assistant tried to pump after this and can not get fluid to either rear wheels, did get fluid to the two front wheels.

Then tried mity vac and got a little fluid out of rear wheels then it stops and can not get any air or fluid out.

Vacumn at booster is 26-30 inches, when removing vacumn booster hose/pipe I hear a hiss. No oil in booster. Swaped booster hose/valve with my parts car no change.

Rear brake hoses have been changed two years ago.

I have done alot of reading and searching on this forum and I am at my wits end!!! Do I have a bad brake booster? bad new master cylinder? should I reinstall old master cylinder? should I swap parts car booster out? Air in system still? I wish I would have never touched these barkes! I have been at this for a week, alternating between power bleeder, mity vac, assistant pump, gravity bleed. Still can not get a firm pedal (it never even seems to build up when pumping!)

help!

you have a PM..

[James L]

Good point on obstruction Junkman! I did remove bleeder screws and poke/clean bleeder hole with a thin wire.

Do I remove rubber hoses at cailper or rubber hose at metal line?

If obstructed, why would I have fluid at wheels when using a power bleeder? more force than normal with an assiatnt pump or mity vac? Could rubber hoses be bad?

[TX76513]

Have you looked to see if any of the soft lines are buldging (need partner) when brakes are applied. Rear lines on W126 are known for this.

[whunter]

Brake:
http://www.peachparts.com/shopforum/showthread.php?t=142411

[clm]

My first thought was bad brake lines- Listen to whunter!!!!!LOL!

Replace them!

[James L]

Thanks for all the replys and phone call to Roy!

This arvo going to try wedging caliper pistons back on each caliper and rebleed as per Roy's excellent suggestion, will also address obstructions and brake hoses. Will report back with results. Thanks

[BoiseBenz]

The reservoir needs to be superfull to overflow into the "Rear" section. You almost have to refill it after every pedal pump.

I learned about 30 years ago that for me the best way to bleed 123 brakes is with the engine running and the two person method. Since that time I have gotten a Motive Systems bleeder and that gets it done if there's only one of me around.

When you bleed brakes with engine running, your helper needs to be an adult. With cars that I could bleed with engine off, which is about any other car in the world, I used my kids when they were as young as about 8, but I don't think I want to be underneath a running vehicle with an 8 year old's access to the controls.

[James L]

Just finished doing what Roy advised. Took off all 4 wheels, on jackstands, took out pins/springs on pads, wedged piston all the way back on all 4 calipers.

I then hooked up power bleeder and bleed each wheel (2Xs). while at each wheel wiggled pads/piston and taped calipers with mallet. did get some air out, and alot of fluid. did this until fluid was solid. went thru another pint of brake fluid. put pins/springs/pads, back in, ect.

NO CHANGE!! all symtoms as above. pedal will still not pump up, resovoir is still as a lake after a pedal pump. (should have bubbles, fluid movement or something)

Did try to bleed before with adult, with car running, did not matter.

Now I am tired, more frustarted and my back hurts, time for a nap.

Thanks for the help

[whunter]

Originally Posted by James L

Just finished doing what Roy advised. Took off all 4 wheels, on jackstands, took out pins/springs on pads, wedged piston all the way back on all 4 calipers.

I then hooked up power bleeder and bleed each wheel (2Xs). while at each wheel wiggled pads/piston and taped calipers with mallet. did get some air out, and alot of fluid. did this until fluid was solid. went thru another pint of brake fluid. put pins/springs/pads, back in, ect.

NO CHANGE!! all symtoms as above. pedal will still not pump up, resovoir is still as a lake after a pedal pump. (should have bubbles, fluid movement or something)

Did try to bleed before with adult, with car running, did not matter.

Now I am tired, more frustarted and my back hurts, time for a nap.

Thanks for the help

It sounds like you may have a defective master..

[James L]

defective master cylinder, that is what I am thinking also. (I noticed made in China on the Meyle box) (Meyle China reman!!)

I should have fluid movement in resovoir regardless of any air. I can take cap off and pump pedal and no fluid moves or shots out. when I have an assistant pump pedal ,no fluid, no air comes out because master is not moving any. But I can get fluid out with pressure from the power bleeder.

On the bench, I noticed the new master did not pump as easily as the one I took off the car.

Next I wll reinstall master (mercedes star) that I had before veneturing into this nightmare.

I have been thru about 8 quarts of dot 4 brake fluid in doing this project so far.

[James L]

Update; Working! I have a pedal and fluid at all four wheels at all times!

I reinstalled original master cylinder (mercedes star), bench bled, power bled, then I had assitant pump to bleed and pedal goes to floor as it should when bleeder opened, it also pumps up!

Brake pedal feel is hard and needs force to brake, about how it was before I started 2 weeks ago. Still needs some tweeking.

What are the symptoms of a caliper needing; cleaning/greasing, rebuilding or going bad? What are the symptoms of rubber brake hoses needing replacement?(2 back hoses are new, 2 front are old).

Thanks for the help.

[Chad300tdt]

In my experience, bad hoses cause fluid restriction, so you can apply the brakes, but the caliper will release very slow or not at all.