Title says it all......83 300D. Was pulling away from a diesel pump, stepped on the brake, there was a pop, pedal went to the floor with only a little resistance, but no braking at all. Fluid is full and not leaking, no brake warning light, but no, as in zip, nada, nothing for brakes.

Any ideas before I get a tow home?

Odd no leaks and complete failure. Check at the top of the brake pedal arm inside the cab for disconnected or broken actuating linkage. Have someone step on the pedal and hold the master, can you feel movement?

If the linkage is good and BOTH reservoirs have fluid, I can't think of anything else other than a failed master cylinder. Check the reservoirs closely especially the rear section.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I have to think the pedal disconnected from the actuating rod if there's absolutely no braking. There are two brake circuits in the master cylinder. Both failing at the same time is unlikely.

Sixto
05 E320 wagon 219k miles
99 SL500 69k miles

If the rear piston sticks forward from your last breaking the piston does not return and fill up with brake fluid.

If there is sufficient gunk at the bottom of the master cylinder that it blocks the ports the piston area will not refill.

You can loosen one of the outlets at the master cylinder and have some one step slowly and see if any fluid comes out. Caution them not to let the brake pedal go to the rear so no air is sucked in till after you get the line nut tighter

Gently tapping the master cylinder might temp free up the pistons if they are stuck.

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84 300D, 82 Volvo 244Gl Diesel

Thanks folks, now that I have calmed down a bit, I thought about it and agree it is weird that it all went at once. I'm leaning towards an issue with the actuating rod, or a stuck master cylinder.

With work and family commitments the next few days, it's going to be a bit before I can dig into it, but I promise I will post what I find and the fix so if (God forbid) anyone else experiences this, they have an idea where to start looking ....

Ok I only got a chance to take an initial look at it once it got towed home. I pulled all 4 wheels and everything looked ok, except the entire inside of the right front wheel and tire were covered with brake fluid. The caliper, disc, and pads are soaked in brake fluid, and the fluid in the rear part of the brake fluid reservoir is about a 1/2 inch below the "min" mark.

My first guess is blown seals in the caliper as the rubber hose, and steel line are dry and intact, but the caliper is covered in brake fluid.

I'm just wondering why that would cause complete brake failure?

Comments or ideas???

Next thing is I'm going to open the bleeder screws and test each caliper to see if it is getting pressure when the pedal is pressed.

Fill the master cylinder and clamp the hose to the leaky caliper. It’s no fun bleeding a master cylinder.

Sixto
05 E320 wagon 219k miles
99 SL500 69k miles

Good idea .....I have only had to bench bleed a master before and even that is a pain in the.......I'm thinking since there is still fluid in the rear reservoir, even though it is low, it should be ok.

Why wouldn't you fill the reservoir up?

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84 300D, 82 Volvo 244Gl Diesel

Well, nothing really. The redundancies in the system would prevent that. If you're panicking or the pedal went all the way to the floor and the system didn't have time to build pressure again, it might have felt that way. Otherwise as you've seen, our w123s have a dual circuit system –the reservoir has a baffle and the master cylinder, two channels.

Pretty scary though.

How on top of your brakes were you? When those seals blow, it's usually due to gunk in the system.

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1981 240D 4sp manual. Ivory White.

Diesel 911......I did fill it up, what I meant to say was since the rear reservoir was low but not empty, it should be ok regarding no air needing to be bled out of the master..... sorry for any confusion.

The front brake pads were replaced about 2500 miles ago and the lines bled. The rears were due real soon and I had planned to bleed the entire thing once the new rear pads were on. I can tell you, I pumped the brakes several times, waited a couple seconds and pumped the hell out of them again, and there was no pressure at all, and not even a hint of braking front or rear ......

What frequently happens when fluid isnt annually flushed, rust builds up in the pistons, they sieze, and you have the last caliper not frozen stopping the entire car.
When THAT piston fails, Voila! No more break function.

Replace BOTH front calipers, then all hoses, then flush entire braking system and see what happens.

PS... when the rear resivoir shows low, it is usually dry, and the fluid meniscus looks like it still has fluid.

Terrible design...

Make sure ALL of the reservoir is at the max line, and you are ok...
Low, means empty...

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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 560SL convertible
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1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

While replacing the hoses is a bit more work they are cheap and even the chain autoparts stores like autozone have brake hoses. All brake hoses no matter where they are from have to me DOT specs. Meaning I have never had any issue on any car with made in China brake hoses.

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84 300D, 82 Volvo 244Gl Diesel

Ok, as promised here is what caused the issue. It was actually a combination problem. The one caliper was definitely bad, but also the piston in the baster cylinder did not retract and was stuck. Near as I can figure, when the one caliper let go and brake pressure dropped, pushing the brake pedal and having it go to the floor overextended the piston in the master cylinder causing it to get stuck. No matter how many times I pushed the pedal, no brake pressure was being generated. Anyway, a new master, a new caliper and hose, full system bleed and I'm back in business.