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Brakes lost pressure

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cjcurtisejohnson
Posts: 74

Brakes lost pressure

Post#1 » Thu Sep 22, 2022 12:25 pm

Team
My G350TD (VIN: WDB46332117087529 7) all of a sudden lost pressure on my brakes. I was driving down the highway and all of a sudden my brakes were really hard to push to the floor like I was getting some kind resistance. I really think it is a vacuum line that came off somewhere, but not sure, or it could be the master cylinder. Does anyone have any idea what this could be, any help would be greatly appreciated.
thanks
Curtis

emoynier
Posts: 506

Re: Brakes lost pressure

Post#2 » Thu Sep 22, 2022 12:42 pm

Check vacuum, then booster and MC.
1997 W463 G300TD

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AlanMcR
Posts: 6772

Re: Brakes lost pressure

Post#3 » Thu Sep 22, 2022 12:46 pm

Sounds like a vacuum problem. Either the piping or the vacuum pump itself.
The pump is on the front of the engine and has a 1/2" nylon pipe heading to the brake booster. There should be side taps along the pipe for gauging the amount of vacuum in the line. A finger test should be good enough to start the diagnosis.
If the problem is in the vacuum plumbing for the lockers, then you can just cap off the feed line to the lockers with a golf tee or pencil.
...Alan

e9coupe
Posts: 354
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Re: Brakes lost pressure

Post#4 » Thu Sep 22, 2022 3:16 pm

AlanMcR wrote:Sounds like a vacuum problem. Either the piping or the vacuum pump itself.
The pump is on the front of the engine and has a 1/2" nylon pipe heading to the brake booster. There should be side taps along the pipe for gauging the amount of vacuum in the line. A finger test should be good enough to start the diagnosis.
If the problem is in the vacuum plumbing for the lockers, then you can just cap off the feed line to the lockers with a golf tee or pencil.

Wow, so they use a vacuum pump to run the system for multiple parts and one of them is the brake booster? Seems like they'd have a better safety setup than that.
Dan
Rocklin, CA
2000 G500 Europa

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AlanMcR
Posts: 6772

Re: Brakes lost pressure

Post#5 » Thu Sep 22, 2022 5:10 pm

The vacuum has to come from somewhere. I have yet to have a vacuum pump failure on a diesel mercedes. 600K+ miles of trying.
It is the hoses and fittings that get brittle and snap. That they have in common with other motors.
...Alan

cjcurtisejohnson
Posts: 74

Re: Brakes lost pressure

Post#6 » Thu Sep 22, 2022 10:22 pm

Thank you Alan, I appreciate your help, have attached a pic where on this Pic would I find the main lead to the lockers? This would help so at least I can start narrowing it down. Now I have the issue of the motor not shutting off when I turn the Key so I have to go under the hood to kill the motor.
Thanks for any help you can provide.
Curtis
Attachments
Vacuum line problem.pdf
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w123_sedan_guy
Posts: 514
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Re: Brakes lost pressure

Post#7 » Fri Sep 23, 2022 2:30 am

Vacuum is used to shut off the engine also. There is diaphragm on the back of the injection pump for this function. Since all your vacuum related systems seem to have a problem I'd look to the main line off the pump first. Its the white/yellowish plastic line coming off of your brake booster on the right of your picture. It will probably connect to a steel line at the front of the engine then to the pump.

Wow, so they use a vacuum pump to run the system for multiple parts and one of them is the brake booster? Seems like they'd have a better safety setup than that.


Most all cars pre 2000's used vacuum to boost brakes and operate other systems. A gasoline engine will produce its own vacuum while a diesel engine will not and therefore there is usually no need for a pump on gasoline engines. All diesels will have some sort of vacuum pump whether electric or mechanical. Nowadays we tell the computer that we desire brake pressure and it is electronically modulated. I could probably make an argument that the old way where your foot is actually applying an indirect force to the brake is safer than than your foot telling a switch to tell a computer to tell another computer to actuate a solenoid or something.
- Kyle

cjcurtisejohnson
Posts: 74

Re: Brakes lost pressure

Post#8 » Fri Sep 23, 2022 4:22 am

Team
So I pulled the big white line directly on the pump to see if there was vacuum and there was none, does the engine have to have a certain RPMs before I get some kind of vacuum. Also does anyone have a document that shows how to remove the Vacuum pump from the motor? It would be great to take it apart to check it out.
thanks team for your help.

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AlanMcR
Posts: 6772

Re: Brakes lost pressure

Post#9 » Fri Sep 23, 2022 6:21 am

If vorking correctly, it will generate lots of vacuum at idle.
The pump is in the form of a plate bolted to the front of the engine. IIRC, it is driven by the same chain gear that turns the injection pump. Take the bolts off and it comes straight forward an inch or two, then up and out.
The older style pumps had a reputation of dumping their broken bits and pieces into the chain mechanism. You don't want that, obviously. Newer gaskets are supposed to be designed with a lip that catches the debris. Remove it carefully.
I believe the newer style pump fits this engine.
...Alan

Mercano
Posts: 431

Re: Brakes lost pressure

Post#10 » Fri Sep 23, 2022 6:47 am

The newer pumps have a different drive system, they have 2 teeth that engage with slots on the injector pump sprocket. The older ones had a cam on the injector pump sprocket and the vacuum pump had a roller system that the cam pushed on. The shape was changed slightly as well, I don’t remember exactly what but I think for oil drain. I think the 603 probably has the older style, so the gasket has to be for the older type.
Gasket for OP's VIN is A6012380780

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