Cannot get air out - Bleeding Brakes W210

[louwdj]

Using a power bleeder, I pushed two liters of new brake fluid through the system and still had air bubbles coming out of the rear calipers. The main dealership never changed the brake fluid in ten years on this 1999, C240 and I decided to dismantle all pistons. Two had a bit of gel and one a bit of sediment. So I consider the (whole day) effort worthwhile. I never had a problem using this power bleeder (at about 20psi) on any of my other (non-ABS) cars. The front brakes had no air bubbles in no time but both rear brakes keep pushing bubbles through. Large bubbles approximately every two inches in the fuild. I eventually started the engine, pumped the pedal, but still bubbles coming through. I have since driven the car and everything seems OK. Any clues why I keep having bubbles coming through on the rear calipers? I am intrigued and may take the car to the dealer, just for a bleed - both of the brakes and my purse!

 

[television]

This is an unusual fault, and it came up once before, but I cant remember what it was.
Bleeding brakes is old hat to many of us, hope that someone can come up with something

 

[brianbrian]

You might find that know you have disturbed every thing, the Master cylinder needs replacing.

 

[television]

You might find that know you have disturbed every thing, the Master cylinder needs replacing.

Thanks Brian, that was it, the pistons for the rear brakes was had collapsed , hence the tank being odd

 

[louwdj]

Thanks guys! I forgot to mention, I only took the pistons out of the calipers. I did not split the rear units and did not touch the master. The car is driving (and braking) normally. It is going to the dealer on Friday for a bleed. They have confirmed that there should be no bubbles - as we all will agree. This may be a bleeding experience in more ways than one! Will keep you posted.

 

[television]

My last post was not clear, it was the piston in the master cylinder for the rear brakes that had failed

 

[brianbrian]

Thank you Malcom, I had the same problem 20yrs ago with a mercedes, brakes were perfect, but decided to change rear flexi hoses, could not blead brakes till i changed master cylinder.

 

[Gt Russell]

are you sure that the pipe thats attached to the bleed nipple is not seeping air ? I mean as the fluid shoots out of the nipple it is pulling air through a gap between the nipple and the pipe if it is hanging down it might not be sealing properly just a thought

 

[popuptoaster]

a bleed after a long time quite often causes seal problems on a lot of cars as they can get damaged running over the ridge that develops as you depress the pedal further than it will normally go.

i dunno about Merc brakes specifically but Sierra ABS brakes for example need bleeding in a certain order and with the ignition on to get all the air out, is there a reccomended way to bleed the mercs ABS brakes?

 

[Xtractorfan]

Use a pipe and bottle on the one of the front calipers and loosen the nipple, then try bleeding the rears, the front may be fully pumped and causing the rear to stop short of expelling all the air..other way is to release the rears and let them drip for a while.. You could also have a perished rear flexi hose..
If the master cylinder doesnt have a dual bleed nipple..front/rear..then try loosenening the rear pipe @the master cyl and pumping..

 

[louwdj]

The verdict is out!

Fluid level has not changed in 5 days, brakes still feels OK. Took it to the dealer today. Fitted pressure bleeder and sure enough still largish bubbles every 1-2 inches from the rear calipers. Technician calls junior (apprentice) and makes him pump the foot pedal, as you would normally do bleeding without a pressure system. The pressure bleeder remains connected. In no time, no more bubbles. Verdict? Known problem, apparently the ABS system somewhere sucks air (which nobody can explain to me where) and by pumping the pedal, the problem is solved.

Furher good news: As a gesture of goodwill, because the same dealership never changed the brake fluid in this car in ten years, whilst in their service care for PO, there was no charge!!

So, next time, despite using a pressure bleeder, I will still need someone to pump the pedal, if the car is fitted with ABS.

Thanks to all who contributed.

 

[television]

Thank goodness, and another " and they all lived happily ever after"