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Old 07-01-2018, 04:41 PM   #6
fdryer
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Join Date: Jan 2006
Location: NYC
Posts: 45,377
 

2003 L-Series 3.0L Sedan
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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