So last week my brakes felt soft, I looked under the hood and realized I was loosing brake fluid rapidly from from the rear compartment of my brake fluid reservoir. I checked connections between the reservoir and the master cylinder which are fine.Fluid appears to be leaking from the connection between the booster and the master cylinder but I cant' tell if it is also leaking from behind the booster. I don't get the "hard" brake syndrome that I read about in other threads, but the brakes do go soft within a day or so. What is the best way to tell if the booster is bad? Is this just a bad master cylinder even if it might be leaking from behind the booster?

__________________
"helga"
1982 300D
white/navy 191k

It's a bad master cylinder. There are no leaks of liquid with a bad booster, just leaks of vacuum.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

That's a bad master cylinder. The seal is damaged and the brake fluid is leaking into the booster. The booster has no fluid in it, just vacuum. Make sure you clean out all the brake fluid from it before installing the new master cylinder.

__________________
1983 123.133 California
- GreaseCar Veg System

PHEW... that's what I thought, any way to tell if the booster is damaged?

__________________
"helga"
1982 300D
white/navy 191k

I have never considered the possible results of a bad "Master Cylinder and Booster", mainly because "most" vehicles are Gassers and the vacuum is created in the intake manifold.....so if some brake fluid were to escape into the vacuum side, it would just wind up in the intake and be burned.....

However, with the Diesel engines having a vacuum pump, that exhausts into the crankcase....can the brake fluid wind up diluting the lube oil?? I assume so, but never really considered the concept before.....

SB

__________________

Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG

Sometimes you can loose fluid from the MC into the booster, pull 'em apart, and inspect. There is an "O" ring between the two. This may be the case if you can't find the fluid that is no longer in the MC.

I'm not sure how brake fluid can make it's way to the crankcase.

If your booster is failing, it won't hold vacuum. It can be tested by evacuating the booster with a mity-vac, and see if it holds pressure.

__________________
83 SD

84 CD

The o-ring between the MC and booster is a vacuum seal and should never see brake fluid. The MC has a small weep hole near the booster junction; unless the rear shaft seal has also failed, fluid should not leak into the booster.

On models with the piston-type vacuum pump, the pump vents into the crankcase, so any brake fluid that enters the booster could wind up in the engine oil.

What pressure?

What’s the best way to do this?

Is there enough space when the MC is removed?noticed today on my CD that the brake pedal is slowly dropping to the floor when pressed, and I’ve lost a bit of fluid (just pressure bled the brakes the other day and set to max).

Thinking to get a new MC and replace.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

If your brake booster is bad, it will be hard to press, if you have a bad brake master cylinder your foot will go to the floor.

__________________
Have you ever noticed that anybody driving slower than you is an idiot, and anyone going faster than you is a maniac?

As long as they would add one additional commandment for you to keep thy religion to thyself.
George Carlin (Wonder where he is now..)

1981 240d (engine donor 1983 240d) recently rebuilt engine hurray! - No more.. fought a tree and the tree won.

pearl black 1983 240d 4speed (Converted!@$$%) atleast the tranny was rebuilt.

Brake fluid ( except for silcone DOT 5 ) is water soluble so in theory you could remove the booster and wash it out. Be aware that brake fluid swells non brake fluid rated rubber so watch for booster diaphragm damage.

At least on my 380sl, the FSM has an allowance of 100CCs of fluid ingress before you have to replace the booster.

I would double check because it may be different on different vehicles.