Brake fluid bleeding

[lgr]

I have a bleeding coming from thecap of the  tandem brake master cylinder of my 1970 280 SL. ´The left reservoir is empety and the right one is full.
Any idea about what is the source of this problem?
thank you in advance for any tip!
Best regards
Luis

[AUPagoda]

Oddly enough I have the same problem. The car has been sitting down since 1991 and i'm slowly going thru her item by item. I had all 4 calipers off for months while they were being rebuilt. Shop took a few days, this busy car fanatic has takens months to get them back on. Anway, mounted them up, started to bleed and flush the remaining fluid. My mechanic kept getting bubbles and the pedal never firmed up. He pulled the MC, did a quick bench test and found the rear section not responding like the front. After a few hand pumps of the piston the prime picked up. Reinstalled but nothing. We used old school method and new school method to bleed with MityVac.

Does anyone have any suggestions for either one of us?

Thanks in advance. 1969 280SL

[mdsalemi]

How old are the flexible lines going to the calipers?  If you put on rebuilt calipers, new lines are in order.
Check the integrity of the hard brake lines as well.  They can and do rust, particularly if something has been sitting since 1991!  Nothing like inactivity to bring out the worst in a car.
Lastly, just replace the master cylinder.  Braking is too important to mess with.

[Benz Dr.]

There's a seal on the piston inside the MC and it's leaking. This the one on the very back and it's often cut or nicked by a bit of rust inside of the cylinder. This seal is there to keep air from entering as you pump the brake pedal.
If this seal is bad, a small amount of air will enter every time you hit the brake pedal if the system is empty or not completely filled. This why you can have good brakes until you change a hose or bleed the brakes. This requires a huge volume of new fluid ( by comparison to gradual brake pad wear ) and the seal won't be able to hold well enough to keep out small amounts of air.
You can pump until the cows come home and you will never get anywhere. Three or four pumps should be all you need to remove small bubbles of air from the system. If you've done ten or more and can't seem to get the air out the MC should be very suspect. 

[mdsalemi]

There's a seal on the piston inside the MC and it's leaking. This the one on the very back and it's often cut or nicked by a bit of rust inside of the cylinder. This seal is there to keep air from entering as you pump the brake pedal.
If this seal is bad, a small amount of air will enter every time you hit the brake pedal if the system is empty or not completely filled. This why you can have good brakes until you change a hose or bleed the brakes. This requires a huge volume of new fluid ( by comparison to gradual brake pad wear ) and the seal won't be able to hold well enough to keep out small amounts of air.
You can pump until the cows come home and you will never get anywhere. Three or four pumps should be all you need to remove small bubbles of air from the system. If you've done ten or more and can't seem to get the air out the MC should be very suspect. 

This excellent possible explanation is validation for the "change your brake fluid annually"; typical brake fluid is hygroscopic; and the added moisture induces rust and--well--there you have it.