losing brake fluid from reservoir, no sign of external leaks. it must be sucking out into vacuum assist, is this a sign of bad master cylinder? brakes and vacuum assist working fine, just losing fluid.

Yes, if the fluid is leaking to "nowhere" it's a bad master cylinder.

You've checked the calipers? If they leak slightly fluid can build up in the dust boot. How much fluid have you needed to add?

If you start the car and stand on the brake pedal does it slowly sink?

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The most likely cause would be the master cylinder leaking into the vacuum booster. Not sure how common that is on a Benz but it DOES happen on any make. Look for signs of brake fluid either below the booster on the engine side of the firewall or on the carpet below the pedal rod - however, the booster can has to fill half way up before it will show signs. If its leaking into the calipers as mentioned it will show sooner or later as this is fluid under full brake system pressure so it'll show eventually.

Dan

It happens on benzes too.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

thanks, just put a rebuilt cly on a few months ago, must be I got a bad one.

This has happened to me on a VW - very similar master cylinder and the rebuilding place had a goof up that day, the very end piece of the master cylinder was placed backwards and that made a leak into the vacuum booster.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

You can loosen the master and pull it back from the booster. There will be a rubber cap on the end that the rod goes through. Loosen the cap and determine whether there is any wetness which will mean that you have found the (perhaps a) leak.

I ended up going wigh a new master from here for more than I wanted to pay but it is working perfectly. I'm hoping to get 100,000 mi out of it and change fluid annually.

__________________
85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

If you find the master has been leaking fluid into the booster I'd suggest changing the engine oil - I don't think brake fluid will do the bearings any favors. I'd also consider replacing the booster, though you might be able to clean it.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.