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I recently purchased an 83 300sd and everything has been running fine until recently. I have done an EGR delete, change the fuel filters, put on a new air filter, air/ oil seperator modification, clean the banjo bolt, and dialed in the VCV in correlation to adjusting the bowden cable to smooth out the shifts. Up until yesterday everything was running perfect. However a slight problem the car had after i acquired it was the brake pedal would sink from time to time at a stop light, but this seldom happened. Well when I went to drive it yesterday the pedal went straight to the floor, the car however would stop fine and with a few pumps would be stiff again, but soon after it went straight to the floor. As far as I can tell there are no leaks at all the fluid has remained the same the entire time. A complete break job was done on the vehicle prior to purchase with the exception of MC and Booster. Whenever the car is off, the first time i hit the brakes the pedal drops straight to the floor, but i can pump it up and it will become firm and remain firm until the car is started. And another odd thing the leads me to believe it to be a vacuum problem is it started shifting hard again as soon as the breaks went out, even though I had it adjusted and shifting smoothly prior, and when I turn the ignition off the car shuts off immediately, or with little delay. Could this be a Vacuum Pump issue, a booster issuee, a MC issue or all three. I am completely stumped.

Please help as this is my daily driver and although it stops i feel very unsafe driving it, especially at highway speeds, any advice would be much appreciated. Thank You.
 

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Sounds like either a brake booster/master cylinder (the MC can leak internally) or a Vacuum issue. The correlation between the hard shifts and the brakes leads me to thing Vacuum leak or the vacuum line to the booster has failed. The check valve? or a hole in the line.
Easy check for that, Remove the booster line at the booster and blow air into it. If you can blow air into the line without building pressure, the check valve is bad or there is a hole in the line. You must replace the whole line as no one sells just the check valve. I would start there first.
 

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Discussion Starter #3
Thatnks for the input, i will attemptmt to check the line and report back with the results. However, when i pull the aux lines attached to the main vac line i feel little pull on my finger with the egine at idle. Could this be an indication of a failing vac pump or is there not suppose to be much vaccum pull present at this junction. Also when i was driving it earlier at times after i pump the brakes or abrupt engagement of the pedal, it would remain firm, atleast until it was released the pedal.
 

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Discussion Starter #4
The thing that really confuses me is from all the research i have dne my understanding is that a vacuum or booster issue is suppose to make the break stiff or firm/ with the exception of booster diaphram or vacuum pump? Idk an and all help is appreciated.
 

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+1 on Chadahar. If it is a vacuum leak other than the check value it is slight. If you have a major leak or pump failure your car won't shut off as well. Get a vacuum pump/gauge device if you don't have one and use it to track down your vacuum leak. Also you can use the vacuum pump tool to test the booster once you disconnect the main vacuum line. The booster if working should hold vacuum. Good luck!
 

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Discussion Starter #6
Thanks everyone for all the help... I slove the problem... It ended up being a combenation of problems, the MC was completely shot as it was kicking up lots of bubbles, there was also a slight vacuum leak on the nipple at the vacuum booster, i sealed it with teflon tape amd tightned it as much as i would go, all is good... I will do a full brake fluid flush tomorrow and hopefullly my brake system will be good as new... Now having reseloved one issue i discovered another one, there is a leak at the fuel line coming off the fuel tank, the very first prelimanary filter, i have plentynoflet over 30R9 fuminjector hose laying around, however i cammot figure out how to get to the tank, any suggestions?
 
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