adjusting the power brake booster push rod

Hello All,
I looked in my 71 Dodge FSM and couldn't find anything about adjusting the push-rod that activates the master cylinder. The brake system is a power front disc brake car. I'm looking to try and fix a situation where the brakes seem to be hanging a wee bit ( like if you were in a driveway and the car doesn't roll down the driveway as it should )..

Cheers,
Troy

I didn't know factory ones were adjustible. but here is how I did it.

basically there is a hole in the master if you look down.
even with fluid in it you should be able to see in to that hole with a good light.

if you pull the master away from the booster, and adjust the rod out till you see the piston in the master move as you push the master back onto the booster.

then turn the adjuster in a little at a time till the piston in the master doesn't move as you put the master back onto the booster.

with fluid in the master, you might even see fluid movement first so be careful how forcefully you put the master back on.

You'll need to unbolt the master from the booster, leave the cap and lines alone, pull it forward enough so that you can measure the tip of the pushrod to the mounting location of the master on the booster's surface/face, I'd recommend a set of Verniers/dial Calipers...then measure from the bottom of the inside piston bore of the master to it's mounting base/face/flange that would bolt on to the booster's surface, the do the math based on the values you have, either lengthen/shorten as needed, .030 minimum to .060 max for Mopar applications

be sure to grip the knurled/splined portion of the pushrod with pliers/vise grips, and turn the hex headed pushrod tip in/out for the desired length

Mike


FYI if your not familiar with using Calipers to measure the bore of the master, you use the section/bar that slides out of the "rear" of the Calipers and the "rear" portion of the main caliper frame to establish your measurement ( the DEPTH ROD as described in the pic below

I have dealt with this in my 70 Charger. I have a 1975 Dart booster and master cylinder on it, 4 wheel disc. The A body boosters have an adjustable pushrod tip on the rod that presses the master cylinder. I have tried to adjust it to get as little freeplay at the pedal as possible, but it always ends up smoking the brakes. I've had to settle with a small bit of dead travel before the brakes engage. It sucks.

I measured to get close... then I ended up pulling the pushrod out of the booster, gently setting the MC in place (keeping it square), then seeing how much farther in I could move the pushrod. Currently have it adjusted to zero, with no apparent brake dragging.

Apparently you're also supposed to be able to look in the master and see a poof of fluid when you press the brake pedal an inch or so if it's adjusted correctly (with everything bolted together).

As previously mentioned You need to have .030 clearance between the pushrod and piston when the pedal is released.

thanks everyone.. I will attempt to tackle this shortly....

Initially, this is what I see. I am going to try and figure out how to measure the distance from the mating face of the MC to the inside part of the piston cup. But, as one can see, it is adjustable....

HMM one of these are handy, we made ours. Hope this helps



http://www.classicperform.com/TechBook/pushrod_adjust_tool1.gif

Back when I did it, I would turn the in (shorten the rod length) for more pedal travel or out (lengthen the rod) for less pedal travel. I would do 1/4 turn at a time in the direction it needed to go, and test it. It may require puling the master off a few times, but it always worked very well. Gene

First determine if the brakes are a wee bit tight without the engine running/ vacuum to the booster...
If they are still tight the booster push rod is not the problem.
Still tight with the engine running, pull the master off just slightly and turn the rod in a bit 1/4 to 1/2 turn and repeat, until the problem is fixed. There is normally enough room that you do not need to remove the brake lines and re-bleed.

This thread has such great information, I had to resurrect it. Plus I am also having brake issues with my recently restored 71 Cuda(disc/drum). When car is cold front wheels spin with light drag, after a trip around the block BOTH front wheels are impossible to spin(locked up).
Do you think this is a booster arm adjustment?
Too much fluid?
Heat?
Help!
Thanks!

Yes, pushrod adjustment. If in doubt, loosen the MC nuts a few turns and see if the lockup stops.

Thank you JK, I'll give that a try.

I have done this twice, and both times I used a contour gauge. They are available at Home Depot and are inexpensive.

Set the gauge in the M/C plunger, then adjust the push rod according to the opposite side of the gauge. At that point, any spacing becomes easy. You can see exactly what you're doing because your measurements are right in the gauge.

Wow thanks! I'll pick one up. What is clearance supposed to be? .30-.60?

The ideal clearance is .020".

if your fluid is clear you can see into the hole in the bottom of the master.
I can actually watch if the piston moves when I push the master onto the booster.
I just adjust based off of that movement rather than any measuring device.
I back it off till it doesn't move any more, then a hair more.

however if you didn't replace anything recently and this just started to happen, I might spend time checking other things.
like the check valve inside the booster?
I had one go bad and suck the brake pedal down.

I have replaced many master cylinders and I never had to touch the adjustment. Have I just been lucky?

[quote=EV271]This thread has such great information, I had to resurrect it. Plus I am also having brake issues with my recently restored 71 Cuda(disc/drum). When car is cold front wheels spin with light drag, after a trip around the block BOTH front wheels are impossible to spin(locked up).
Do you think this is a booster arm adjustment?
Too much fluid?

I a 71 Cuda come in doing this, changed everything did same thing you describe after hot, people told guy it was the headers, the line lock, had the proper .020 pushrod clearance, it was the reman master cylinder causing it. Put a new bendix on problem solved, but this master looks wrong for a resto. Might be something better out there now

^^^^@440-6 you could be right. I recently replaced master, with reman. unit. But I also didnot adjust booster arm.

Any ideas on a correct master for 71 hemicuda?

How do you subscribe to a thread? I don’t see the link?

I bought a reman for my 71 6bbl cuda at carquest, looks right and it works, original cover fits it. They are now owned by advance so that may have changed, or it could change daily. Most stuff is cardone brand, hit and miss. I rooted thru the local carquest warehouse and got that master ,a pair of 1970 date coded calipers, and a set of 1971 date coded calipers, that were NICE, no rust pits. People are sending there old cores in I guess, worked for me.