Sudden loss of pedal, not really a spongy feel. The pedal must be pushed to the floor to get anything, but can lock up the wheels at that point. After some searching it soudns like this is not air in the lines, but the master cylinder. When pumped the brakes get a little spongy and then lose it. When the car is off and the brakes are pumped, they gain nothing-from a post this sounds like the booster is gone too.

I see no signs of leakage and when I went to check the fluid level the cable to the hood broke. It's open, I can pull the plastic latch in the grill all the way out, but the hood itself won't come up with some combined yanking.

I will replace the hoses and bleed this weekend.

I wasn't working on the brakes recently, but I was in the engine compartment... is there an easy to knock off hose that could cause this?

__________________
1981 300D 205,000 mi.

Usually, a sudden loss of pedal corresponds to a leak in either the front or the rear system. Once the m/c runs out of fluid in that system, the feeling is one of "1/2 podal". So, before I'd condemn the m/c, thoroughly check the wheels for any signs of brake fluid. If it's one of the wheels, you'll find it pretty quickly............and you'll also find one of the chambers in the m/c out of fluid...........typically the rear chamber (which you cannot easily see from the driver's side of the vehicle).

If you are sure that there are no leaks and both chambers of the m/c are full...........the problem is the m/c itself.........one of the seals has failed.

I'd get yourself a MityVac from your local Sears. I recently bought one for myself, and I must admit that it's an invaluable tool when trying to diagnose a lot of problems on these older cars.

If you have one, I'd check the vacuum pressure coming off the vacuum pump. It should be giving you at least 17hg, anything lower and the pump is suspect. While you're at it, I'd take the hose off the brake booster, and use the mityvac to pump it to roughly 20hg. See if the brake booster holds 20hg for 10 minutes.. mine only lost 1/2 an hg in the course of 10 mins.

After that, plug the lines coming off the main vacuum splitter (there should be two splits, if this is a W123), and see if the main vacuum line (connected to the pump) holds vacuum. If it doesn't, it could be a check valve, or the master line itself.

I had a lot of vacuum brake issues recently, myself.. and I know how frustrating they can be. My MityVac only cost me $35, and it has already proved its usefulness to me.

__________________
_______________________________
(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

You don't have any symptoms that will allow you to chase a vacuum problem. It's a low pedal...........not a hard pedal. It will lock up the wheels.........no booster vacuum wouldn't allow that unless you were a gorilla.

Here's an interesting question then,

Last night I replaced my front brake pads (one was about 1mm thick remaining! ) And afterward I pumped the brakes up to get the pads in place, and the pedal was spongy, I could push it down, it'd hold firm, but would then sink to the floor if I kept pushing harder. It felt like the Master Cylinder was "leaking down" as I'd call it. My pedal on my car has never been "hard" but now its softer....I inpected under the car and at the wheels after several "sinkings" of the pedal to the floor, not a drop of fluid anywhere, and the booster works fine. So I am guessing the master cylinder is dying....I went for a drive and the car stops fine with not all that much pressure. If I jam on the pedal while driving it will lock ALL the wheels up right away, it still has massive braking power. And as of this morning, the pedal doesn't seem to sink to the floor anymore...it stays pretty firm and gives good braking response....

The hoses on all 4 corners are not even 2 years old, all lines are ok, and I fully bled the system with 2 liters of new ATE Dot 4 fluid in September....so no air in the lines either. Reservoir is full. On my dad's SD, it takes very little effort to lock the wheels up. Could I have a worn master cylinder + iffy calipers? Thats my guess. I may replace the MC soon since they are cheap....and I am paranoid.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

The '86 had all kinds of leaks for nearly one year. First one rear wheel would leak.........rebuilt that..........then the second would leak..........rebuilt that.............various problems on multiple occasions. All of these situations required exercising the brake pedal to push fluid though the lines.

Finally, it started to exhibit the same symptoms as you describe. A feeling that it won't quite hold in a given position with constant pressure. Bit the bullet and replaced the m/c.........problem resolved.

Apparently, the theory is true. Moving the piston and m/c seals all the way through the stroke and into the areas where they don't normally travel will stir up any contamination/corrosion in the m/c and damage the seals. This strongly supports the use of a pressure bleeder in lieu of the pedal.

Whoops. Probably would help if I read the symptoms a little closer. I saw "brakes" "booster" and my mind immediately went "hey, I had brake issues last week! It's a vacuum problem!'. Sorry.

(Though, I still stand by my statement that the MityVac is an invaluable tool, and that everybody should own one.)

__________________
_______________________________
(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]

Currently my brakes are working well now, they seem to have recovered, but if it gets at all spongy again I will replace the master cylinder asap. I just don't want to do it now because its freezing outside and I just bled my brake system to the tune of $20 3 months ago.

I tested them today by pushing harder and it didn't sink to the floor, and I jammed on them with dry pavement and it locked the wheels. So I am calling them good for now.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

My 90 560SEL decided to lose its brake fluid and the pedal goes to the floor - almost NO braking power. Can't be driven for sure.

It happened over the course of a day & 1/2 - good brakes in the a.m., "weak" last evening. worse this a.m. and not functional tonight (bear in mind its the wife's car & description - I did pilot it last Saturday and all was normal).

She claims no noticeable fluid leaks at work and there are none in the garage here, so...

Where is it going ? (Sorry, MC and booster technicals are beyond me...I studied this thread but got lost...) - keep in mind it has ABS..

Getting it carried over to my Indy in the a.m. but would like to understand...

Help appreciated

Best regards,
Kevin

__________________
"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew

I hijacked a thread.

Sorry,

Kevin

__________________
"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew

AND you hijacked it with a Gasser question... well technically, you asked a hydrolic question and since you DO own a Diesel I'm willing to let it slide...
also, it's more than fair to hijack a thread that is 3 months old...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

...not hammering me.

Correctomundo though, our brake systems are the same except for vac source....

Doing a search, this thread was as close as I could find to my situation...

Any ideas?

Kevin

__________________
"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew

The OP problem sure sounds like classic mc failure, especially if it is locking up the rear brakes only.

Tom W

__________________
[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.

... that the reservoir is empty - that's a lot of fluid gone quickly...

I'll post my Indy's findings.

Regards,
Kevin

__________________
"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew

only two reasons for low brake fluid.
1 a leak
2 SEVERELY worn pads all around displacing fluid.
check the inside walls of each tire for signs of fluid pouring all over them. then look under the car for broken lines, rust, split flex hoses etc...
last, look at the brake booster, it can show a leak of fluid from the MC...
report back.