on my 72 datsun 620 i have the brake booster(the round thing behind the master cilinder) disconnected from the vacume line.because when its connected and engine runing the brakes grab hard and the pedal goes to the floor.what could be the problem.i think i need a new brake booster.
what do you guys think.where could i find one.

I suspect that you need to get yourself a workshop manual for your model. Then you can look up the troubleshooting section & this will help to answer the majority of your questions. The illustrations will also show you how things work, & how to fix many things yourself.
It's the best thing you will ever buy for your truck.

Sounds like the "reaction disc" is missing. It normally live on the end of the booster piston.

Has the booster/master cylinder been pulled apart lately?

If it has, pull off the booster and shake it. If something sound like it's bouncing around in the booster, that's what it will be. Carefully pull the piston and rubber seal out of the booster housing, if there is no rubber disc on the flat end of the piston. shake the booster around more until it falls out the hole. Re-assemble the booster with the reaction disc stuck to the end of the piston with a light smear of grease.

Cheers,
Chris

ps. A hole in the booster diaphragm normally results in a rise in idle speed, these are the only 2 things that go wrong with a booster.