Where is vac leak? Weak power brake, delayed engine shut off.
300SD 1985 CALIF.


If I am in stop and go traffic, the brakes work perfectly the first time I step on the pedal. The second time I have to step on the brakes harder. Third or fourth time, no power brake. Stops if I press pedal hard. So checked vacuum. 15-20 at idle. Down to 10 after 1st stop. Takes 30 plus seconds or more to go back to 15-20. If I make 3-4 stops without waiting, vacuum goes down lower each brake application to 5 and pedal is hard and stops like manual brake.

After 1-2 minutes of driving, the brakes return to normal, vacuum slowly comes up to 15-20 and I repeat the cycle.

Also, diesel engine takes 30 seconds to stop by key off. Correlates with vacuum increasing to near normal slowly. Fuel off lever underhood stops
engine immediately.

Checked a 1990's Chrysler Minivan at check valve. Different results from Mercedes. Vacuum at idle 20+.
Vacuum pressing brake, down to 15 but comes up to 20 in less than one second when release brake no matter how many times I press brake.

Decided to change brake booster with Cardone brand rebuild. Behaves exactly the same.


Checked vacuum engine RUNNING at idle. At exit of pipe from vacuum pump 20+. Break seal and re measure. Vac up to 20+ in less than one second.
Vacuum on branches of main line near check valve 15. Vacuum on line off check valve 15.
But it takes 30-60 sec to come up to 15 from a cold start.

Checked if vacuum holds in several lines off black main line with engine OFF using vacuum pump.

Black Main line from vac pump to vac booster. With ends and branches plugged. Holds vac at 15+ more than one minute.
Brown line near check valve. I think for engine shutoff. Holds vac 15+ more than one min
Red green line near check valve Holds vac 15+ more than one min
White line from check valve. Holds vac 15+ more than one min
white blue or clear-blue line. Goes to branches underhood. See diagram. Holds vac 15+ more than one min
White or clear line near check valve, Goes to branches underhood. See diagram. Vac does NOT hold. Able to pump up to 5 or so and decays gradually. Leaks vacuum in area of black part behind right headlight which controls EGR. Do not know if this is normal. I feel that leaks in these small lines would not be enough to run out of power brake vacuum but I am no expert.

Brake booster itself, Up to 15 but leaks down to 10 in a minute. Seal to vac gauge not the best.

Where Is Vacuum Leak?
Is it check valve?
Is it Bad power brake booster?
Or something else?
What else to check?
Thanks.

Vac diagram first picture.
Line from vac pump to start of black main line to booster. Disconnected. 2nd picture
Brake check valve and many branches off. 3rd picture.
Testing vac from tube off check valve. 4th picture.
Checking vacuum with vac pump and golf tees to block outlets.

Attached Thumbnails

         

Your door locks work off the vac pump in the trunk, not from vac under the hood. Completely separate system.

Still having same problem. Probably brake booster or vacuum pump. How to find problem?

Lose power brake assist when braking 3 or more times with no time between. Measured with vacuum gauge from port on check valve on black main line to brake booster.

Result is vacuum goes to 5 or less when braking 3 or more times and slow build up of vacuum to above 15.

Takes 30-60 seconds to get up to 15 or more vacuum after braking 3 or more times.

Have used test hose between vacuum pump and brake booster with no joints. No change in vacuum loss. Have used test hose with Chrysler check valve in it and no other joints. No change in vacuum loss.

So I suppose the problem is the vacuum pump or power booster.

How to check vacuum pump? Have done this: Measure with vacuum gauge at exit of pipe from vacuum pump. Result 22. Break seal and re measure. Vac up to 20+ in less than one second.

How to check brake booster? Connected vacuum gauge to port on check valve. Leaks vacuum down slowly over several minutes with engine off. Same when creating vacuum with a hand pump with engine off.

Thanks

Right under where the Steel Vacuum Line screws into the Vacuum Pump is a brass colored Check Valve.
Unscrew that from the Vacuum Pump and spray it out with WD-40 (some parts inside are Plastic; I would not use Brake Cleaner in that area).

After that do an inspection of the valve and see if any of the inside parts are broken.

I am saying this because when I lost Vacuum with similar symptons cleaning out that Check Valve fixed it.

Others have found the Check Valve broken inside.

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84 300D, 82 Volvo 244Gl Diesel

Have you tried putting a vacuum guage on the booster and then operating the brakes to see what happens?

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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

Thanks. What were your brake symptoms? Also, I have measured vacuum at the outlet of the vac pump with a vacuum gauge on the pump outlet in picture 2 of the first post. It measured more than 20 with engine at idle and when I removed the vacuum gauge and put it back, it measured more than 20 almost immediately. Do not remember if I checked it with engine running and then stopped. What symptoms would a broken vac pump check valve have?

Yes. See 4th picture in first post. I checked vacuum with engine running by putting the vacuum gauge on the port on the white check valve. Did not matter if other vacuum tubes were connected or all vacuum connections were blocked with golf tees. Same result. Also same result with home built test pipe and Chrysler check valve and no other joints, measuring vacuum through a port on Chrysler check valve.

Yes, tested vacuum applying brakes 4 or more times with no pause.

On engine start, vacuum rose slowly over a minute to 20. Engine at idle, applying brake once lowered vacuum to 15 and held steady if foot on brake was steady. Lifting foot off brake pedal, vacuum lowered to near 12. Second brake application, vacuum lower, to 8-10, lower lifting foot off brake. 3rd brake application vacuum lower to 5-8 or less but not zero. 4th, vacuum lower, near zero. Pedal gets hard, and must press hard with both feet to get normal stopping distance. Takes about a minute for vacuum to rise to 20. Engine shut off delayed until vacuum is about 10 if shut off valve is connected.

I've never monitored the vacuum on a booster before but I'm thinking you have a bad booster. I don't know of anything else in the system that can consume vacuum at the level that the booster does. I can't see how it's a bad pump if the vacuum returns.

__________________
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

I had hard braking; it took a lot of effort to brake.
In my case the Vacuum Check Valve was only durty.

I have not had a broken Check Valve. The description of symptoms for that may be in the DIY section.

DIY Repair Links
http://www.peachparts.com/shopforum/forumdisplay.php?f=82
http://www.peachparts.com/Wikka/DoItYourSelf

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84 300D, 82 Volvo 244Gl Diesel

before replacing the booster, double check the seal in the booster, if it is original it could just be a $1 fix
goes between the booster and the master cylinder for the brake fluid thing

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Do not think it is rubber seal between booster and mstr cylinder. Because booster holds vacuum fairly well when I pull vacuum on the fitting on the white check valve. I do not hear hissing or bubbling and do not see water going fast inside the joint when I poured water where the rubber seal is with vacuum at 15.

you could have obstructions in the main vacuum line itself, that is a free fix it. Usually in the orifice that connects the extra vacuum lines, worth checking.

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Good point. To answer that, made 2 test pipes. First with no joints at all. Then with only a Chrysler brake booster check valve in it. Used screw clamps to make good seal at brake booster and vac pump outlet tube. Measured what happened to vacuum with the tube with Chrysler check valve and observed feel of the brake pedal and drove car.

Power brake behaved exactly the same with either test pipe or original main vacuum line with either all branches of vac lines connected or all branches blocked with golf tees. Vacuum measurements were also the same with either test pipe, or original main vacuum line.

Meaning when engine starts, it takes 60 seconds for vacuum to build up to near 20. When braking, vacuum goes down with each brake application. After 4 or so, vacuum is near zero. Brake pedal hard. Then it takes 60 seconds or so to build up vacuum near 20 and get much power brake assist.

So you have already disconnected from the booster side and plugged the extra holes and checked the vac pump and it was good? I think 17 or better is a good reading. Since you blocked the extra holes in the main vac line and same results, to me it sounds like the booster. I had the same symptoms in my 123. Try replacing the seal in the booster first. $1 fix, better than $150, just to see if it will work. I wish I had tried the seal first before replacing my booster. But once I did the brakes worked like they were supposed to. I had hard braking like a big truck w/o power brakes and slow shut off too, but mine is a 123, same principal I think tho. Slow shut off in mine is a leak in the door locks but your system is different for the door locks.
Another thing to consider is the brake hoses, they can get clogged too. They will deteriorate from the inside out, and look good from the outside. Could have crud built up inside the lines

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Thanks for suggesting. Yes 15-20 end of main vac line. 20+ at metal pipe from vac pump where pic number 2 shows. I thought so too. Already installed a Cardone brand reman but was no better than old booster. I guess any part can fail at any age. Will try it. However does not seem to have a big vacuum leak where seal is. When I made vacuum with a handheld vacuum pump, It held near 15 for a minute but did leak down over 5 min. And I ran soapy water around where the seal is and found no bubbling noise, and no sucking of water in to seal area. Plugged off all branches with golf tees. So mine is not door locks only.
Old car, anything goes. Brake pedal gets hard after first brake application and it does stop normal distance on the road. Just have to press very hard.