brake booster and m/c, or just m/c?
 

I’ve read some of the threads about brake fluid getting sucked into the master cylinder, and I suspect this is happening with my recently-acquired 300TD. I filled the reservoir several weeks ago when the red warning light began to flicker-the level was just low enough to make it come on I guess-and all was ok until today, when it came on again a few blocks from work. Then at the same time the brake pedal got very hard and I had no power assist so I had to really stand on the brakes to stop the car (this was two corners from the parking lot, at low speed). I pulled into the lot and the car wouldn’t shut off, so I used the lever under the hood (which worked instantly).

My questions are (because I’m confused by the various threads I’ve read): am I correct in assuming that there’s a connection between the sudden loss of power-brake assist and the inability to turn the engine off with the key? and do I need to replace only the master cylinder, or the master cylinder AND the brake booster? thanks for any advice!

Both of those functions rely on vacuum.

You first priority should be to determine the reason for the apparent loss of vacuum.
What year is your 300TD?

It your master cylinder was bad your foot would have gone to the floor because the brakes would of been ridiculously soft and vague and car would of kept on going. So the master cylinder is fine.


I'm leaning towards loss of vacuum not associated with the booster.

it’s a 1983

this is where I’m confused..I’ve read in several places that mysterious loss of brake fluid can be due to it being sucked into the booster..there was no mention of a soft pedal or loss of braking power. The fix in this case was to replace the m/c.

I understand that-theoretically-there may be two separate issues here, but they both happened at exactly the same time so it’s logical that they’re related, no?

Fluid loss can be anywhere in the brake system. As the pads wear, the pistons in the calipers move further out and the level will drop as well. Usually if the MC is leaking into the booster you'll see evidence of brake fluid at the bottom of the booster. Look for "sweating" at the seam.

Hard pedal and lack of shutoff indicate a failure of the vacuum source of a very large vacuum leak in the system somewhere. Check the vacuum at the pump. You should have 20" of vacuum or better if the pump is working. If you have adequate vacuum at the pump, you likely have a failed brake booster or a failed check valve in the brake booster line. If the diaphragm fails, it's just like opening up the vacuum line to the pump, you won't draw vacuum anywhere else in the system.

In order to draw logical conclusions, you first need to understand how things work. If the two "issues" are the loss of power brake assist and the failure of the engine to shut down, you have two symptoms of the same problem. The underlying problem is loss of vacuum. You need to identify the cause of the vacuum loss. The vacuum pump would be a logical place to start. In fact, the check valve on the vacuum pump inlet is a very common cause of sudden vacuum loss.

You need a hand vacuum pump to diagnose anything. That will give you a vacuum gage as well. Choices are the Mity-Mite ($40) or Harbor Freight ($25 w/ brake bleed kit). If you just want a quickie stab at a fix, cap off the 2 vacuum tubes going to the trunk lock actuator, since that is the biggest culprit in "low vacuum" issues. That quickly fixed the 1984 300D my son brought home a week ago for "door locks erratic and engine sometimes doesn't shut off". My lifestyle has not been materially-degraded from lack of an auto-lock trunk. But I don't know if your wagon is identical back there.

If that doesn't fix 'er, there are a gazillion posts on trouble-shooting a "low vacuum" problem. Before blaming the pump on the engine, first connect only a vacuum gage to the main tube off it, with nothing else going to the car.

Here’s my thread on this issue. I had a hard to press brake and no shutdown. I mistakenly thought bad booster. Then I saw the what looked like corrosion/paint failure in the booster and I changed the mc. Silly. My mc leaked a tiny bit. And my booster was still good. I just had a bad check valve on my pump at the end. But, it was pretty easy and cheap to swap the 35 yo stuff out.

http://www.peachparts.com/shopforum/diesel-discussion/377572-1982-300sd-booster-vacuum-checkvalve-issue.html

If you are worried about fluid loss separate the booster and mc and get a look in there for leakage. My leak was so small I never refilled fluid between flushes.

On the plus side I changed a rotten sway bar bushing while the booster was out. Also removed an old rat nest. Got to fully flush everything.

Forgot to suggest - if you want to confirm your theory about sucking your brake fluid through the booster, crack the vacuum line between the booster and the vacuum pump and look for fluid. It should be dry.

have ordered a Mityvac, will report back once I have it and have done some testing. Thanks for all the advice!

ok, got the Mityvac and started checking things. Testing at the hard (metal) line that comes directly from the vacuum pump, I got about 22” of vacuum (with the engine running). So at least the pump is good, and the check valve at the pump would have to be good too, right? Or am I mistaken? If I’m getting good vacuum at the end of the hard line I thought that meant the check valve was also good-please tell me if that’s wrong!

I reconnected the flexible black booster hose to the hard line. There is what I guess is a valve in the middle of the hose that goes to the brake booster. It has an arrow on it that points to the vacuum pump. At that point there are two vacuum connections that go to other stuff on the engine. I checked each of those with the engine still running and got nothing. Then I disconnected the hose from the brake booster and attached the Mityvac to that end-still with the engine running, and got nothing-until I pulled off the second of the two small vacuum lines in the middle of the hose (the one closer to the booster) and put my thumb over the tee-that caused the gauge to slowly (20seconds or so) climb to about 15”, and when I took my thumb off the tee, the gauge stayed at 15. I was confused by the reading, as I expected the reading to be the same as what I got at the pump, even without covering that tee with my thumb. But I’ve read that that valve moderates the vacuum going to the engine parts, so maybe it’s affecting my reading in this case?

But why don’t I get vacuum at the booster end of the flexible hose with the engine running? I pumped up the Mityvac on each of the two lines that come off that hose at the valve (the lines that go to engine parts) and I was able to get it up to around 15 or 20 with a few pumps (if my memory is correct). Do I need to pull off those lines, cap both of those tees and test the booster end again? I should mention that the flexible line going to the booster looks almost new, so it was replaced recently. Maybe the booster was acting up and the PO though that line was the problem, who knows.

If I connect the Mityvac directly to the brake booster and try to pump it up with the Mityvac, I get nothing even with lots of pumps (maybe 40 or 50). I can hear a tiny puff of vacuum when I pull the fitting out of the booster, but the gauge barely moves perceptibly so it’s probably only 1” or less of vacuum. do I need to duct-tape the Mityvac connection at the booster really securely and pump it like, 500 times to get the vacuum up? Shouldn’t it start rising after 40 or 50 pumps?

Sorry for all the questions, I’m obviously having a hard time understanding this stuff. I just don’t want to buy a new booster and the hassle of installing it, if that’s not the problem. There are many threads on forums, and articles about vacuum issues, but I feel like often they are written by and for people that already have a pretty good grasp of the system, not for a novice. I have a Haynes manual but I haven’t found anything in there about the vacuum system other than one paragraph describing the vacuum pump..no diagram of the lines, or detailed testing procedures. It seems like the vacuum system should have an entire chapter devoted to it.

since no one has responded, I’ll ask again: if I’m getting good (22 or 23 “) vacuum at the end of the metal line coming off the vacuum pump, does that mean the check valve that’s right on the pump is ok?

If you are getting good vacuum directly at the pump, the pump is good. If you're getting poor vacuum with the lines reattached, you have a vacuum leak somewhere. If you have very low or no vacuum, you have a BIG leak somewhere.

Nobody answered because it isn't clear exactly what you tested. Photos always help. Your "at the end of the hard line" sounds like you connected that alone to the vacuum gage, with no other "vacuum consumers" connected, which simply proves your pump is good. Connect consumers 1 at a time to find the culprit. You need a set of rubber vacuum caps and such to isolate tubes. Be careful with the 2 small plastic nipples off the main vacuum line (at check valve) since they become brittle and snap off easily (I replaced a broken one w/ Cu tube and epoxy).

I don't think that applying vacuum w/ a hand pump to the booster is a definitive test. I think it is normal for the inlet valve on the booster (opens as you push the pedal) to leak slightly, and perhaps faster than you can hand pump. You could probably test it w/ an electric vacuum pump. Another component which leaks even faster (by design) is the VCV on top-rear of injection pump. That is a somewhat quirky design which lets the diesel sort of simulate the intake manifold vacuum of a gas engine to produce a shift signal to the transmission.

thanks-the reason for my question was that I thought there was a check valve right at the pump (in other words, at the pump end of the metal line that comes off the pump-it’s the one that sometimes fails and the bits get sucked into the pump). Someone had linked me to another thread where they suspected their brake booster was bad, but it turned out to be that check valve. It sounded like-from that thread-that it was possible to have good vacuum at some points in the system but still have that check valve be faulty.

It’s confusing me though because sometimes people say they had good vacuum and they mean when pumping it with a mityvac, but that isn’t made clear so it sounds like they’re saying the car’s pump is producing vacuum-know what I mean?

If I remove the hose from the brake booster and cap it, and the car then shuts off normally, that would definitely point to the brake booster as the culprit, yes?