Hi Guys,

Help! I drove to the grocery and my brakes got hard to press and then the engine wouldn't shut down. I looked under the hood and all my lines looked ok. I limped it home.

Mityvac at the ip vacuum T showed 3 in Hg. I measured 20 in Hg at the metal line directly to the pump. I disconnected the booster and plugged up the T's with golf T's and I could measure 3 in at the hose end at the booster with the vacuum reservoir connected. Then I plugged off the reservoir and the vacuum at the booster fitting went up to 20 in Hg. I mityvac'd the reservoir and it holds...dang near got tendonitis but it does hold.

Checked the seal between the booster and master cylinder. Squished up but I greased it with vacuum grease and put it all together. Still couldn't shut down or get brake action. No hissing anyway.

So naively I said, well it's the booster. Big leak in there. Pump is working. But in the back of my mind I'm wondering why the pump needed the reservoir blocked off to work? I couldn't get the engine to shut down with only the booster blocked off, the reservoir connected and the shutdown line connected.

Weird. So given the nice 20 in Hg number on the pump I ordered a new booster. I figured heck, toss in a rebuilt master cylinder while I'm at it. They're on their way from the big cheap place on every corner.

But I started reading threads here about boosters and I started wondering about the check valve. No way I thought. I have 20"Hg at the pump. No way I have a bad check valve. However the fact I had to plug off both the booster and the reservoir to get the shutdown to work was still burning in my head. Didn't compute.

So for grins I pulled the check valve. I should have done this first.

I got this much out of the hole.



Dangit!!! Did I get everything out? Would this explain my wonky vacuum behavior? The pump was sucking but maybe that's because there are multiple check valves in it.

I have a couple of days for the booster and master cylinder to come in.

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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

Did you check the brake booster to see if it is leaking down after the engine is shut off? You do this by putting your vacuum gauge on the vacuum line T closest to the booster while capping off the other vacuum line. Start the engine and you should see at least 20" of vacuum. Shut the engine off and the gauge should continue to read at least 20" for quite a while (several days). The other check is to shut the engine off and you should be able to pump the brakes 3 or 4 times and not have a hard pedal which indicates you still have vacuum assist. While pumping the brakes you should see the vacuum gauge go down.

Thanks,
I could never get the gauge to read up to 20 inHg when the booster was hooked up. The gauge mounted on the T to the ip/tranny controls.

I had a pal push the brake up and down and I put my thumb over the port on the booster. I could feel pressure build up under my thumb with the pedal going down and vacuum on my thumb (a little) as the pedal was released.

I have a booster on the way if a new valve won't fix it. That valve looked bad so I know it'll have to be replaced. All the parts should be here in a few days. I'll follow up with the fix. I plan to replace things one at a time so I know what went wrong.

First the check valve, then the booster and master cylinder.

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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

Your gauge should be hooked up to the other T that provides vacuum for your door locks and HVAC system. This is the T that is closer to the brake booster. While performing the test the other T should be capped off. After performing the test you will need to shut off your engine using the engine shut off lever as there will no vacuum going to the engine shut off valve. Actually 15-20" of vacuum is sufficient. Replacing the brake booster can be a chore - check it out every way you can before replacing the booster.

Ok thanks. There are two Ts - one to the ip, transmission and I think it goes to the HVAC. It's a w126 and the locks have their own electric vacuum pump in the trunk.

The T nearer the front is the shutoff. I've plugged them all and I cannot build much vacuum when the booster is hooked up. I'll start by replacing the damaged valve. The booster does look like a pain. Thanks for the heads up.

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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

Ok, I got my check valve. I put it in. Everything works great.

But I bought a rebuilt booster and a master cylinder.

Should I replace the booster and mc? Aww man decisions decisions.

I also bought a sway bar bushing and two upper control arms because it would be easier to get them in with the booster out. But now I don't need it out...

So the lesson is check the check valve before ordering the booster.

Well, the car has close to 200k on it. And the parts were cheap. So here goes, I'm going to replace them. Now I sound like one of those cheesy mechanics "I replaced it all because it'll all eventually fail and now you can be secure knowing you have great brakes. "

Oh man. I'm so dumb.

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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

Having vacuum but low vacuum is a symptom of the check valve that you show in the pic. When that happend to me it was the same check valve being full of gunk that caused the same symptom.
Hosed it out with WD-40 and it has been working fin since 2008.

If it was mine I would be curious to see if the check valve could be re-assembled if some way could be found to hold the shiny plate in place.

A lof of people have not been able to find the parts when the check valve comes apart.

On the Piston typ vacuum pump there is 4 disc valves inside beside the outer Check Valve. Which is the reason why you don't loose all of the Vacuum when the check valve goes south

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84 300D, 82 Volvo 244Gl Diesel

Also if you were taking the vacuum reading off of the vacuum line that goes to the white Valve on the Fuel Injection Pump (the one that controls the vacuum to the transmission) there is a restricted orfice in that line that reduces the vacuum.

So between the low vacuum due to the Check Valve issue and the restricted orfice that would lower the vacuum reading from that line.

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84 300D, 82 Volvo 244Gl Diesel

Finished the install. Took about four hours. Not too bad. I'll post pics later. I'm beat. Still need to flush the brake lines. Got a new mityvac silver line.

Yeah I think I had vacuum but it took a long time to pump down. When I replaced the little $100 valve it pulled down very quickly. You know, I really thought about getting a little snap ring and trying to JB weld that valve back together. It was pricey for what it is. However I figured I would treat myself to a new check valve...and booster, master cylinder, sway bar bushing, upper control arm.

The big takeaway from pulling the booster is use a 13mm gear wrench to take out that blind nut up in the corner. It really made it a lot easier. It has a really fine pawl so you can work in the limited space...two clicks per swing but that is all it took. I used the ones that don't have the switch on them, you just flip them for forward and backward.

Ok, more in the morning. Good night.

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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

Ok, here are some images of the job.




booster looks awful but it still worked. Unfortunately I misdiagnosed the check valve for the booster. Lesson, just because you have vacuum and you cannot pump down the booster, doesn't mean your pump is working and the booster is bad.



use a 13mm gear wrench to get that last bolt. this is a w126. It is a little hard to get your hand up there.


ahhh, room to work

Now to get that sway bar bushing. Easy now.
\



the old bushing was pretty crushed and rotted out in the hole. Unfortunately my clunk as I pull out of my driveway is still there. So far I've eliminated the lower ball joint, the brake support rod, tie rod, upper control arm, sway bar bush. One day I'll figure it out. Maybe a sticky shock or the spring scuffing on the rubber spring shim. No worries.

Okay, so there it is. Car brakes, shuts down.

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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 don't understand how a bad check valve would keep the power brakes from working or lose vacuum so the engine won't shut-off. It would have to be leaking to the outside, like a bad gasket seal or cracked body. As I understand, in all cars, the purpose of the check valve is to hold vacuum in the booster in the event the engine dies (lose main vacuum). That allows at least one more power-assist brake cycle in an emergency. Otherwise, it should work as long as the engine keeps providing a steady vacuum, check valve or not.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I don't really get it either. What was weird is while hooked to the booster and reservoir I got almost no vacuum in the broken state. When I disconnected those and plugged the hose with my finger or measured directly at the pump I got 20 in HG. That's why I thought my pump was good and the booster was bad. But it was the opposite. After swapping the valve it all worked.

No leaking pipes...believe me I checked really hard before I started this odyssey.

I ended up changing the booster and mc because they were cheap and I wanted to do some front suspension work. Car also has 200k on it.

Oddly the engine sounds different now. The pump seems to have a low pitched growl at mid throttle. I guess changing the check valve makes it work less?

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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 think with the booster hooked up, or the reservoir for that matter, the pump needs to move volume. With my thumb blocking off the booster line the only volume to pump down is the tube. Gauging directly at the pump even requires less volume to be evacuated.

Perhaps, just a theory here, the parts rattling around in the pump were able to block off the small orifice in the first internal check valve ( one of the three button like internal ones) and choke out the pump under high flow conditions encountered when pumping an empty reservoir or booster.

I'm an old electrical engineer. It is like having a signal source that has high impedance and can maintain voltage under low current draw but loses voltage when you draw too much current.

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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

You said cheap for the power brake vacuum unit and master cylinder. How much are they out there? The price I see is over $600.00 and I think that is pretty steep.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Well, I didn't want to get into it because I didn't buy it at Pelican. It cost me $137 after I turned in the core. A remanufactured unit if that bothers anyone. Free shipping and some sales tax. oh yeah, forgot to mention, the big chain store gave me a $35 gift card with the over $100 purchase. So it was really cheap.

The bottom line is it was easy enough to replace that I figured hey, its primarily labor at this point. And yes, it was a little tough getting the little bolts on and off, especially the one in the corner. I have hands like E.T. . If you have big mitts you may have to take some stuff off like the cruise amp. But with slender hands and a gear wrench it was 7/10 on my tough to put together scale.

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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