long story short: a friend was driving my car and said she had to slam on the brakes and pedal stayed on the floor (she had to lift the pedal from the floor with her foot) now the brakes go to the floor.

when i press the pedal, i can hear a hissing sound coming from the vacuum pump in the front of the engine (possibly from the check valve).

the brake fluid was low. i filled it up.

there are no visible leaks in the lines all the way back.

the master cylinder is not leaking into the booster (pedal is not hard--it goes to the floor)

i tried bleeding the brakes, but they would not firm up.

i feel like the car is acting like the master cylinder is bad, but i'm thrown off by the hissing coming from the vacuum pump.

is the only connection between the brakes and the vacuum system the booster?

i'd appreciate any thoughts on this mystery. thanks

--austin

Sounds like a vac leak of some sort, you'll need to test as much as you can.

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83 SD

84 CD

My money's on the master cylinder. The good news is they're cheap and easy to change.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

Agreed, sounds like a master cylinder....if the booster/vacuum was bad, the peddle would be hard...

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i appreciate the responses everyone. is there any connection between the vacuum system and the brakes besides the booster?

No.
But, if you have a Manual Transmission the Clutch Cylinder uses Brake Fluid from the Reservoir so a leaking Clutch Cylinder system can drain part of the Brake Fluid Reservoir.

Also if the Seal on the Back of the Master Cylinder goes Brake Fluid gets inside of the Brake Booster and you will not see the leak.

If the Brake Fluid was low and the Master Cylinder was pumped it is now likely that there is Air inside of the Brake System and it needs to be bled.

In the pic 6 and 7 are rear Seals on the Brake Booster end of Master Cylinder. Notice that #27 is a Pin that keeps you from sliding out the front Piston of the Master Cylinder of you decided to take it apart (you would need to remove that pin).
This is a pic of a Master Cylinder that is not for ABS Brakes.

Getting a decent rebuilt Master Cylinder can be a Chore.

I got a CarDone one and while it took along time the Pedal would start to creep down. About 1.5 Year later the Pedal went to the Floor.
I could not find the Receipt to exchange it under the Warranty and I bought a new one from FENCO that was made in Taiwan and have had no issues. This was a New one not a FENCO rebuilt one.
At the time the price was a good one but now they have gone up.

(The Master Cylinder Rebuild kits tend to match the Cost of a rebuilt Master Cylinder or cost slightly more.)

Later I found the Receipt and got an exchange but left the New Master Cylinder on the Car.

They are now selling New Master Cylinders made by Matellie (spelling?). One of the Ebay Sellers that answered Me said they are made in Italy. But, I think there needs to be some more verification on that. I have not read of someone using one yet.

Attached Thumbnails

 

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84 300D, 82 Volvo 244Gl Diesel

ok, sounds like most people say master cylinder--which has been my inclination. maybe i'll find that i have 2 problems. a master cylinder and a vacuum pump problem.

i think i'm going to put a new master cylinder in it tomorrow, and see where that gets me. i'll let you know!

yeah it unfortunately sounds like you have two problems. luckily it is easy to diagnose and fix/replace the vacuum pump or check valve if that ends up being a problem.

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1981 NA 300D 310k miles

When you do the Master Cylinder...
it MAY need to be BENCH BLED....
do not think you can accomplish that in the car... it looks simple .. but you need it on a workbench or held in a vise .. gently.. .
Napa has the metric plugs and lines to fit the M Cylinder..
Just follow the instructions that come with the M Cylinder...

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1980 240d ,

chain elongation, cam marks reference:
http://www.peachparts.com/shopforum/diesel-discussion/10414-help-i-need-check-stretch.html

http://www.peachparts.com/shopforum/diesel-discussion/305365-9-degrees-chain-stretch.html

evap fin cleaning:

http://www.peachparts.com/shopforum/diesel-discussion/156207-photo-step-step-post-showing-w123-evaporator-removal-1983-240d-1982-300td.html?highlight=evaporator

A/C thread

http://www.peachparts.com/shopforum/diesel-discussion/297462-c-recommendations-mb-vehicles.html

Yes, DO bench bleed the new master. That is NOT a maybe. While it's possible to bleed it on the car without the bench bleeding it's a real stinker.

Also, I have the vacuum pump from my '85 OM617 300SD (I'm running manual brakes on the S10 so I didn't need it) which you can have for the shipping. It worked when I pulled the engine from the donor so it should be a good one. Hopefully the guys here can tell you if my pump is an exchange for yours.

Dan

I have a Meteli master cylinder. It's been in for over a year and is working well so far.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

thanks everyone. Dan Stokes, I am familiar with bench bleeding, and that it is a must on master cylinders. i appreciate the offer for the pump. i'll do some research to see if it fits my car. i'll PM you.

update: i replaced the master cylinder tonight. oreilly had a rebuilt one locally so i didn't have to wait.

i bench bled it, put it in the car, and there is some improvement. the pedal has begun to firm up a bit. however; the left rear will not bleed. i get nothing but air. it won't even gravity bleed. but there are no leaks in that line front to back.

i bled all the other calipers no problem.

i also still have a sucking sound from the vacuum pump when i press the brake. now, i know i've asked this, but there is no way a leak in the vacuum system could be putting air into my brake line right?

the weird thing is that i don't have any other vacuum issues so, i'm not sure what to make of that.

if it turns out to be the vacuum pump, someone is going to have to explain this to me. the two systems aren't even connected, so how can a vacuum leak lead to pedal going to the floor?

hopefully this will help someone else in the future!

btw, i put new rubber hoses in the rear, so they're not old and swollen. is there a cut off valve to the rear? (proportion valve i think?)?

What are you bleeding the brakes with?

Old fashion way, mityvac, pressure bleeder?

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