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Old 07-01-2018, 11:17 AM   #1
Figman
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Default Intermittent vacuum leak

I own a 2000 SL2 automatic with 130,000 miles. A couple of weeks ago the car suddenly began running rough and I had to keep the rpms above 2000 to keep it from stalling. At the same time, I lost all power assist to the brakes. Classic vacuum leak, right? Well I checked all the hoses, the brake booster and the check valve on the booster and could find nothing wrong. I started the car and drove it several miles with no issues. The engine was fully up to temperature. idle was normal and the brakes worked fine. A couple days go by and the same symptoms return. I was barely able to get the car home and I had no power assist to the brakes. Again, checking for vacuum leaks turned up nothing. I am afraid to drive the car now, but it is needed every day. Any assistance would be greatly appreciated.
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Old 07-01-2018, 01:01 PM   #2
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Default Re: Intermittent vacuum leak

The rubber tubes are all still supple and not dried out or cracked?
If you have a bit of tube slack, i'd cut off a chunk and reattach the tubing anyway.
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Old 07-01-2018, 01:19 PM   #3
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Default Re: Intermittent vacuum leak

The hoses all look good. I pulled them off one at a time to be sure of a good connection. I can't understand why it is good one time and bad another.
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Old 07-01-2018, 03:09 PM   #4
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Default Re: Intermittent vacuum leak

Any engine misfire when brake pedal is pressed on? Spongy or a stiff
pedal? Might be the brake booster check valve or the booster itself.
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Old 07-01-2018, 04:02 PM   #5
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Default Re: Intermittent vacuum leak

The check valve on the booster appears to be OK. I don't know how to test the booster itself. The thing is, the problem is intermittent. Sometimes it runs and brakes fine.
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Old 07-01-2018, 04:41 PM   #6
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Default Re: Intermittent vacuum leak

The reason to suspect the vacuum line, check valve and vacuum brake boost unit is your description of sudden brake pedal loss when braking then it returns. Unless you have a massive brake fluid leak (check brake fluid level), loss of braking with the brake pedal mysteriously going to the floor then operating normally suggests something wrong somewhere. Rubber ages and dry rot sets in with visible crack lines. If in doubt, replace the hose. Check valves inline with the brake vacuum hose tends to last but they too can fail. The check valve operates in one direction - vacuum sucks open the valve and when the engine stops, the valve closes. The built up vacuum in the round brake boost unit holds enough vacuum for several power assisted braking emergencies if the engine stopped while driving, enough reserve to brake to a stop. If the check valve or brake boost unit fails, power assisted braking isn't available and braking will require every bit of leg muscles to brake.

While the metal vacuum brake boost unit rarely fails from internal failure or exterior corrosion (rust), they can fail. Severe rusting on the exterior of the round boost unit behind the brake master cylinder may be a hint.
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