no power brakes

I had to replace my master cylinder because it had started to leak. ever since then I have no powewr assist on my pedal. before this my 66 bonneville had very good stopping distance for such a big car. now it is pure manual brakes and takes forever to get slowed down. I have vacum in the canister and I have vacum out the back of the carb to the canister. I bought a new check valve but no difference. the new master cylinder is supposed to be for power brakes and had advance auto recheck the part numbers. I am stumped. thanks for any help I can get.  

It is possible that your power booster went bad. I have seen this on a number of occasions in the early 80's. A lot of times when youreplace something the next thing in line will go bad. i would recommend replacing all the hoses and wheel cylinders since this has to do with brakes, your only means of stopping besides that Lexus in front of you.  

I concur replace the booster and I would, at least, replace the flexible lines, and fluid (use DOT 4 since it has a higher boiling point).  

I did replace the booster. that made no difference. I even took that one back and the second one did not help. the booster has vacum in it and holds vacum. pull on the check valve and I get a hiss of air rushing in.I also had turned all the drums and rebuilt the cylinders and put on new shoes. thanks for all the ideas I think it is something in the master cyl that is not right but I don't know what it is. I flushed the lines and bleed the lines so many times I went thru 3 bottles of dot 3. I might have to rig up some chutes if I get out on the interstate.  

Once you have eliminated all of the other possibilities, then whats left must be the problem.  You have tried, and done everything I would have done.  Let us know if the new master cylinder does the trick.  

Did you bench bleed the new master cylinder?? You can do it on the car but easy in the vise before you install it. Usually the tubes and adaptors are included with the remanufacture master cylinder. Maybe the Chinamen don't include them now. You will never get brakes if you don't do it. To bad you didn't go with DOT 5. I have had it in vechicles I did 20 years ago and is clear as the day I put it in, and it's harmless to paint. It's not nearly as expensive as it was back then.  

with the engine off.. pump the brake pedal a few times....   take your foot off for a minute...   pump the pedal again..

does the pedal pump up???  get higher with each pump????  if so,....    you might have air in the brake system...  

66... did you convert it to a dual system????  

did you verify that the master has the same size bore as the one that came off.?????

now.. to test the power brake booster...

pump the brake pedal to release any left over vacuum...      keep your foot on the brake pedal.. while you start the car..  does the pedal drop???  yes or no???  

when the engine pulls a vacuum.. this while your foot is on the pedal.. will start to assist. and pull the pedal down..  

I usually like to work alone, but if you have a helper when you bleed the brakes it help's alot. You can lay down there and open and close the bleeder valve while your helper slowly pumps the brakes. When bubbles stop , and straight brake fluid starts flowing - have the guy hold the brake all the way down and tighten the bleeder. After you repeat this on each brake you should have good braking pressure. Just keep the master cylinder full in the process.  

Don't forget to start at the farthest away from the master cylinder and work your way back to the drivers side front wheel. You may have to do the rears more than once. But first bleed the master back into itself. I take it this car has a single type master cylinder. Or has it been converted???