Bendix Power brake booster push rod length

[MrDee]

I have 69 440 GTX with power disc/drum brakes with a Bendix vacuum booster. Do I use the .910/,920 push rod length that is shown in the Kelsey vacuum booster section of the FSM (no spec in the Bendix section) or, do I set a .020 gap between the booster push rod and master cylinder plunger? If I use the .910 dimension, there will be a .160 preload on the master cylinder plunger. And, the only way to achieve the .020 gap between the plunger and pushrod is the grind .250 off the push rod adjuster tip. I've researched the chit out of this online, but it's obvious the people answering don't have actual early Mopar knowledge. I'm hoping to tap into someone here that knows the correct answer.

 

[RemCharger]

Is the pushrod not adjustable?

 

[MrDee]

Yes, but after screwing it all the way in, it's still .160 too long to achieve a .020 gap. It's fine as is to achieve a .910 length.

 

[RemCharger]

Ok the small amount of gap is the important part. Can you post a pic of the master ? Makes a guy wonder if something is up with the depth of the hole

 

[MrDee]

I have two masters. I have the one that was a correct right port replacement that was on the car when I purchased it. The other master is a factory correct right port p/n 2226821 rebuilt by White Poat Restorations. Both have the same plunger depth of .750. If I use the fsm dimension of .910, that's where the .160 preload comes from.

 

[RemCharger]

Then if I'm reading this properly, the pushrod grew after the 1st master was removed.

 

[MrDee]

No, the pushrod was normally set to the longer dimension. I actually have two factory Bendix vacuum booster, both were set long. As I a mentioned, the only way to achieve a clearance between the plunger and pushrod is to screw the adjuster all the way in and grind .2 off of it.

 

[RemCharger]

So you've tried sliding the masters on to verify they are being held out/ away.?
Again, some pictures may help .

 

[MrDee]

Yes, of course, and I have a tool to measure mc depth and booster pushrod length. What do you need a picture of, a Mopar master and Bendix booster?

 

[RemCharger]

I'm curious if the piston is sticking out of the master? Or has the pushrod come out of the booster somewhat? Or what the pushrod tip looks like. Without pics it's just imagination.
The last one I did was an off the shelf 70 booster, with an over the counter raybestos master, and required a very slight adjustment - worked perfect. There was no need to be concerned with the factory spec # s whatsoever.

 

[MrDee]

Here you go. The booster that's on the car has the pushrod screwed all the way in and .2 ground off the end. This modification gives me .020 clearance between the pushrod and master plunger. The rusty booster came on my car and the plunger is sitting about .940 in length.
The two master cylinders are an original p/n master and a factory replacement master that was mounted on the rusty booster. Both have a plunger depth of .750.
If I use the .910 fsm length, the plunger pushes in .160 and the plunger will sit flush with mounting surface instead of protruding like it does.
Logic would say put it together like it was with the .910 dimension, but that preload the master. Is that facory? Grinding 2 tenths off the end of the adjusting nut to achieve a .020 gap doesn't make sense.

 

[RemCharger]

Ah okay.... yes I believe the piston should be flush, that's your aggravation. So your thoughts would be correct. Is there a small bolt on the bottom? That should be there to retain the piston. I haven't dissected one of those, but I suspect possibly the end of the bolt is not shaped properly? To hold the piston in further.. if your old one has the bolt , take it out so we can see?

 

[MrDee]

Yes, they both have an identical bolt. No damage to either one.

 

[Jerry Hall]

If the plunger on the master cylinder is flush do you have the needed clearance? The plunger sitting flush with the master cylinder face is not going to apply any brake pressure. I say bolt it up, just like the old one came off and see what you have. Some master cylinders have a small tab to keep the plunger flush with the face.

 

[MrDee]

No, there's .160 of push from the booster pushrod to get the plunger flush. So the master is preloaded by .160.

 

[Jerry Hall]

I recently had to replace the bendix booster on my Superbird. I didn't measure anything but adjusted the push rod on the rebuilt booster like the one I took off. After reassembly I had slightly more petal travel than I wanted before the brakes actually started applying. I then extended the push rod length one more turn to get the petal feel like it needed to be. Just my 2 cents.

 

[MrDee]

Do you still have the old booster, and if so, can you measure the pushrod?

 

[Jerry Hall]

The old booster is on its way to Booster Dewey for a rebuild. Sorry.

 

[MrDee]

Not a problem, I've actually finally figured it out. I found this old Master Tech video on the operation of our Master Cylinders.
Chrysler Master Tech - 1970
Volume 70-9 Brake Hydraulics
The operation is basic of course, but, the video shows a couple photos of the primary and secondary pistons when at rest. The rubber pressure cups are positioned just behind the compensating ports, and the filler ports are halfway open. These facts means the master cylinder piston must be even/flush with the mc mounting surface, not protruding out 160 thousandth like it does in an unmounted- at rest position. By setting the booster rod length to .910, the mc piston is flush with the mc mounting surface when it's mounted.

 

[RemCharger]

Thank you. Very interesting. This is the MC that came with the booster I put on for a guy last summer. It doesn't have the retaining bolt on the bottom.
Not sure what it's technically from