adding power drum brakes

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   I have a 69 Rebel SST 343 auto. It originally had manual drum brakes. I put new everything on all four wheels, and it worked fine for manual brakes. I decided to add power brakes so my girlfriend would feel more comfortable driving it. I got a used bracket from a salvage yard, cleaned it all up and painted it. all the pins are tight and free in the bracket. When I installed it I noticed the push rod from the petal was a little short. I had to adjust  the brake light switch in to make the lights go off. I purchased a new power booster. I hitched the vacuum line to the spot where the PCV valve was to test. I have 20+ inches of vacuum at all RPMs. When you shut off the engine and pull the one way valve from the booster, you can hear that it does store vacuum. If you shut off the engine and place pressure on the brake pedal, and restart the engine you can feel the pedal go down a little.  The brake are not power! they are worse than no booster at all. another thought was the length of the push rod to the master. I tried testing by adding washers between the master and the booster, then between the bracket and the booster, nothing helped.

I'm out of ideas.

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thomson29 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Fan Joined: Oct/23/2016 Location: Barre, VT Status: Offline Points: 3	Post Options Post Reply Quote thomson29 Report Post Thanks(0) Quote Reply Topic: adding power drum brakes Posted: Oct/23/2016 at 1:44pm
	I have a 69 Rebel SST 343 auto. It originally had manual drum brakes. I put new everything on all four wheels, and it worked fine for manual brakes. I decided to add power brakes so my girlfriend would feel more comfortable driving it. I got a used bracket from a salvage yard, cleaned it all up and painted it. all the pins are tight and free in the bracket. When I installed it I noticed the push rod from the petal was a little short. I had to adjust the brake light switch in to make the lights go off. I purchased a new power booster. I hitched the vacuum line to the spot where the PCV valve was to test. I have 20+ inches of vacuum at all RPMs. When you shut off the engine and pull the one way valve from the booster, you can hear that it does store vacuum. If you shut off the engine and place pressure on the brake pedal, and restart the engine you can feel the pedal go down a little. The brake are not power! they are worse than no booster at all. another thought was the length of the push rod to the master. I tried testing by adding washers between the master and the booster, then between the bracket and the booster, nothing helped. I'm out of ideas.

farna Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Online Points: 19691	Post Options Post Reply Quote farna Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 6:38am
	The pushrod hole in a manual drum master cylinder is deeper than a power master. Get a power master cylinder if you can't get the pushrod depth right. I have used a steel plug in the master, after figuring out what length was needed with a loose fit wood dowel. Don't leave the dowel in even if it works perfectly -- it will degrade and crush over time. Replace with a steel rod (cut off bolt works). Some boosters have an adjustable rod, but they won't adjust enough to make up for the manual brake MC. The manual MC has a deeper rod hole so that the rod can have a retainer, not needed on power since the rod is part of the booster. The brake pedal may go closer to the floor than you'd like. The power pedal has a different shape than the manual. You can fix this by making a hack saw cut 3/4 of the way through the pedal arm about an inch below where the pushrod connects. Cut on the driver's side of the pedal arm, then bend the cut closed and weld. To straighten the pedal make the same cut right above the pedal on the firewall (opposite) side, close and weld. That 1/16" cut will move the pedal up an inch or more when closed, bringing it up off the floor. It will be higher when brakes are off also, but so is the factory power brake pedal. You could of course swap the pedals if you find a factory power pedal. I've done it both ways.
	Frank Swygert

thomson29 Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Fan Joined: Oct/23/2016 Location: Barre, VT Status: Offline Points: 3	Post Options Post Reply Quote thomson29 Report Post Thanks(0) Quote Reply Posted: Oct/24/2016 at 8:08am
	I had a good hard petal, it did not feel like it went too far down. I removed power booster, and manual brakes work much better. I will put it back on an try what you said and see. I may put a headless bolt in there to shorten the hole.