Driving home tonight, the brakes suddenly failed (or the "power-assist" suddenly failed)--I had to stand on the pedal to stop the car. Exciting through stop-and-go for about 10 miles! (85 300CD).

Got home and car would not shut down--had to use the lever on the engine. Doors won't lock.

There appears to be no vacuum at all.

Can this be a leak? Or is it a vacuum pump failure? I can't find any loose hoses or connections. (I have had a minor leak for quite a while--the vacuum would disappear over night and I had to manually unlock the trunk or doors--never bothered to trace that leak down).

There is no negative sound that I can hear in the vacuum pump--no metallic mechanical failure to hear.

So, questions. Do vacuum pumps commonly fail so suddenly? Or can a leak have suddenly gotten so bad that the brakes or shut-down wouldn't work?

Not sure how to trouble-shoot the vacuum pump. I have a Mity-Vac but haven't used it on the vacuum system yet.

Thanks for any suggestions.

Thanks, TXBILL.

I don't know what I did, but I fixed the vacuum problem. I first unhooked the big line at the brake booster and felt it--a definite vacuum, but I didn't think it was very strong. Then I unhooked the two tri-tees between the booster and the pump, thinking I would plug them with golf tees. But I got worried I was losing track of what went where, so I plugged them back in. In the process, I pulled the other ends of two hoses out of their fittings by the firewall. I stuffed those back in.

Then I thought I'd start all over, so I started the engine to get a "null" state. Suddenly, I clearly had vacuum to the brakes. I cracked the line at the brake booster--definitely vacuum there. I hooked it back up and took the car out for a run. Everything works fine--in fact, the transmission shifts were very nice (I'd been starting to wonder about them). Key off and an instant shut down.

Back at home, I checked over all the fittings. I can't see any that are obviously bad, but they're all old. I noticed that the PO had put a hose clamp on one of the lines to the firewall--one of those that I had accidentally pulled out. Why a hose clamp? I suspect a previous problem. I had just shoved it back together, and it seems to be working now. I'll give it a couple days try-out.

At any rate, I think I'll replace all the weird fittings--they are nearly 20 years old. Anybody have the part numbers for all of them?

I'm quite amazed that what had to be a small leak could have taken enough vacuum to keep the brakes from working.

Thanks,
Ron Dwelle

I would invest in a second vacuum gauge (two if you do not have one already) and go ahead and pulmb (TEE) them into each of the Tri-tees off the main line (pump to brake booster) and secure them to the fender area and do some monitoring of the gauges With the system operating normal, record these values. When a problem develops, you only have to raise the hood and look at the gauges. No distupting the lines needed to get a reading. Based on my '85 300CDT, The main line pump to brake booser with the tees disconnected and a vac gauge on) should be near 22"Hg. the aft tri-tee is 17"Hg. and the fwd tri-tee is 11"Hg. (memory questionable on the fwd tee reading). I need to recheck and record these numbers so If they are different I will correct this statement. With this setup you could move lines (under the hood) to see if a connection is loose without temporarily fixing the problem before you can test anything. go ahead and get some vacuum line and start replacing the rubber connections. Old rubber line leaves your hands a mess so replalce them as you go. remember also that several components in the vacuum system operate on a proportional bleed-off so every branch of the system will not hold vacuum.

Attempting a file attachment with this also. Hope that it works.

Here is the correction to the vacuum values that I posted from my ’85 300CDT. Going from memory didn’t do too well. This is the diagram of the hookup. Vac pump to brake booster was with fwd and aft Tee system lines not connected (one port plugged and gauge on the other.

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