Reasonable assumption to me but let others confirm. A better test might be with the booster line capped do you have normal vacuum level restored? Using your gauge. Long term master cylinder leakage back into the booster because of a defect rear master cylinder seal. May have damaged something in the booster remember. You also have a loss of brake fluid issue and the car is new to you.

Oops, that was my link. I’m sorry I have been distracted with other things.

To answer your question, yes this exactly what I saw doofus. I measured good vacuum from the pump directly with a mityvac. That is I disconnected everything (no Teed off fittings) and connected my mityvac directly to the pump with a single hose only going to the pump. I got vacuum. You can read my thread I forgot how much. Then, when I put it all back together I got no vacuum. I thought there was a leak right? Makes sense that an upstream leak would make me lose all vacuum. But no I blew it. I got fooled.

But the problem is to check your booster for leaks witha mityvac will give you carpal tunnel syndrome. It is a BIG volume to pump down. I eventually did it to prove my booster diaphragm was good.

The problem in my case was the check valve. My theory (you can read my thread) was that the pump could evacuate a small volume like a tube hooked to the pump but not a big one. Also, I think pressing the brake makes you lose air, that is why you only get a limited number of presses under vacuum for power breaking. There is also a reservoir down the fender that is an additional volume that needs to be pumped down.

So I wrongheadedly replaced my booster and master cylinder when I only needed a check valve. It sounded like you had the same symptoms so I suggested my thread. It takes two minutes to unscrew the line at the pump and inspect the first check. There are more inside but the first one at the hose is what failed on my sd.

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

i can’t unscrew the line right at the pump until I get a crowfoot wrench-I will report back once I look at that check valve. I did some other tests today but they’re probably inconclusive. I’ll explain just in case:

I disconnected the main line at the booster and capped it off. Halfway between the pump and the booster there is a check valve with two connections. I tested each of them in turn, with the engine running and the other one capped off. The one towards the front of the car had 23”of vacuum about 30 seconds after starting the car. With the Mityvac still attached to the front connection, I un-capped the rear connection and reattached the vacuum hose that had been there, shut off the key and the engine stopped after about 8 or 10 seconds. If I attached the Mityvac to that hose (the one that goes to various things on the engine, other rubber vacuum tees, etc) and turn off the key, I can stop the engine that way, pretty much as soon as I start pumping and the gauge hits 5” or so.

Then I attached the Mityvac to the other connection on that valve-the rear one towards the brake booster (which was still capped off, as was the front connection). This time with the engine running it only read 9” of vacuum, and it took several minutes to get there. Since everything else was capped off, why the big difference between the two connections? (23” front, 9” rear) Is that valve supposed to drop the vacuum that much, even with nothing else connected?

Also, with the gauge attached to the front connection and the rear one capped, if I reconnect the main hose to the brake booster I only get barely 2” of vacuum after waiting several minutes.

I understand I still need to look at the first check valve (at the pump), I’m just posting this info in case anyone spots something else that might be a clue. How do I test that check valve, though? Is it basically a case of “if it’s intact and not gunked up, it’s good” or is there a way to test it? Like maybe you can blow through it one way, but not the other way?

ok so I removed the check valve and this is what I got. I stuck a magnet into the hole to get the small bits. I noticed other people got one more part, that plastic (?) piece-I’m not sure if it’s in there or not, or how to remove it..I’ll try feeling around with a piece of guitar string or something perhaps. If I blow compressed air into the hole will I damage the pump? What about sucking it out with a vacuum cleaner?

https://imgur.com/gallery/r85Nk

you can’t tell from the photo but the valve body itself is empty, there’s nothing in it and you can see right through it.

oh yeah, the hole in the pump makes a right-angle bend so you can’t exactly reach in with tweezers or anything. Can I try to suck that piece out with a vacuum cleaner, or will that wreck the pump? What about blowing w/compressed air? Any suggestions welcomed!

Yep, the plastic disk is gone. It may still be rattling around in there, or it may have crumbled and passed through the pump. You can try a vacuum cleaner, it won't hurt anything, the water lift on even the strongest vacuums is far less than what the vacuum pump creates.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

new check valve ordered, should have it by Wed. The old one seemed to have something on the threads when I took it out-is it supposed to have some kind of sealant or Teflon tape or something when it’s installed?

Hey that looks like my valve. I think you have the same problem I had. Hope you read thru my thread and didn’t run off and change the master cylinder or booster!

I don’t recall any sealant. Also I think I got the plastic piece out by attaching a 3/8” vinyl hose to my shop vac with lots of duct tape and sucking it out. The passages are really small in the pump and I don’t think it’ll get ground up. There are at least two more check valves in there. It was two years back and i forget.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

yes, I read your thread-it was very helpful. Definitely prevented me from rushing out and purchasing a brake booster. After the new valve is in I should at least have working power brakes, then I’ll figure out if I need a new master cylinder-pretty sure I need at least that as brake fluid is disappearing and not leaking at the wheels. In the course of investigating I also discovered that the vacuum lines attached to the valve in the middle of the main vacuum hose (the one running from the pump to the brake booster) were reversed. Hopefully once I have proper vacuum I’ll see an improvement in shifting-it wasn’t horrible, but wasn’t great either. Thanks!

the vacuum cleaner worked..I fabricated a nozzle by cutting down an old funnel and taping it to the hose. It took a few tries and I was about to give up when suddenly it sucked out this part:

https://imgur.com/gallery/1NIjC

getting the threads started on the new valve was awkward because of stuff in the way..I finally attached it loosely to the metal hose and used that to position it in place, then with two fingers gingerly got the first turn on it. After that I removed the metal hose and finished tightening it with the 22mm socket. Got the rest hooked up and voila-brakes work and car shuts off. Thanks for all the contributions to this thread!

Glad it worked out. I recall the valve was a bit pricey. Stung a bit.

And I still don’t quite get this part. Your vacuum pump measures out good but you proved like me that it was a destroyed check valve and nothing else. Funny but hey, happy we have working cars.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles

I’m sure as someone said (maybe it was you,or maybe it’s in another thread?) it’s a question of load, kind of like like when you test a battery and it’s showing 12 volts, but as soon as you put it in circuit where it’s got to do some work, it drops to 5 volts or something. Or like in an AC circuit, that check valve is like a big capacitor for a fluctuating voltage, perhaps. At least that kinda makes sense to me..

Had this happen with my SL. There's an economy gauge which is really just a cheapo vacuum gauge. Makes identifying vacuum leaks very easy.

When I would hit the brake, the gauge would drop down which revealed to me that there was a tear in the brake booster diaphragm.

Try this instead. T up your mity vac to the vacuum line off of the brake booster. Start the car.

Have someone push down on the brake pedal and hold it there. The vacuum gauge should drop, but quickly recover. If it drops and doesn't recover, that's a sign of a bad brake booster.

Be forewarned, however, that these brake boosters generally last the life of the car. The only reason they fail is if the master cylinder has been vomiting brake fluid into the booster inner chamber.

I would remove the MC and inspect the inside of the brake booster. If it has liquid inside of it, you will then know what to do.

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-Typos courtesy of my mobile phone.