Free play in power brake systems.

[Kern Dog]

In my ongoing quest to get the brakes to feel right in my red car:

I have tried a multitude of combinations. Factory Cordoba 12” front disc/ rear drum with a 75 Dart power booster, 11.7” rear discs from Dr Diff, manual master cylinders with the 4 wheel disc, back to the Dart booster but with bigger front rotors with smaller calipers, another manual MC, hydroboost, then it all finally was right once I swapped in a correct type dual diaphragm booster for a 1966-70 B body. Mine feels right and stops as well as a newer car.
I’ve been helping a buddy with the blue Satellite seen above. He had an aftermarket booster that wasn’t working right (providing little boost) so I swapped in the same type booster I used on my car. Braking is excellent but like other power brake B body cars, there is a bit of a dead spot from the pedal at rest to where the pedal moves enough to feel the brakes responding. I accepted it as normal but the owner wants it to respond quicker.
The fluid level was fine:

Not full to the rim but not low enough to be an issue.
I pulled the MC forward and tried backing out the nub on the pushrod.

 

[Kern Dog]

The threaded nub they use on these aftermarket units is shorter than the stock ones I have here.

Look at this size difference.

Now this wouldn’t matter much if the pedal free play were minimal but it isn’t with this car. Backing out the screw/nub extends the pushrod length and shortens the travel needed by the pedal. This short of a nub really limits the amount it can be adjusted.
Here are a few of my stock ones.

These all vary in length, leading me to think that they are adjustable for a reason. They are probably adjusted to meet an acceptable standard.
The short length of this one limits the amount I can adjust it. It was already more than halfway backed out when I checked it. I’m going to ACE hardware to see if I can buy a longer one to try. Based on a standard I’ve lived by, a bolt needs to engage by at least a depth equal to its thickness. This is metric and measures about .22, clearly less than 1/4” like the stock ones. I’d feel better if it engaged the main pushrod by double that.
More to report, I’ll be back…,

 

[Kern Dog]

Ace hardware had some Allen head cap screws that matched what I needed.

The pushrod that stays in the booster is threaded deep enough.
I measured the old booster to see how far the nub sticks past the mounting face.

Now I’m aware that a tape measure is somewhat crude compared to a real tool that would measure to the thousandths of an inch but I thought it would give a ballpark idea.

It was too far out. The master cylinder wouldn’t seat without pressing the piston. As it turns out, I forgot that I took the threaded nub out of the gold booster and just screwed it back in enough to stay in place. I now had no reference point and had to trial fit and adjust the nub until the master cylinder would seat.
Now the pedal gap is very short and the brakes don’t drag but the feel is weird to me. It seems like it is more grabby suddenly instead of in a linear fashion.
Ultimately it is the goal to set it to where the owner likes it, not what I like.
Since it is adjustable, there is no harm to leaving it as it is for him to drive and assess.
I’m still curious to read input from others that can comment on their amount of free play before the brakes react.

 

[Kern Dog]

for those that like to split hairs, the size and shape of the end of the pushrod nub can be debated.
Stock:

The aftermarket nub:

This is the nub from the first booster that was in the car when it got here:

Not really rounded despite the recess in these master cylinders having a sort of female dome.

The ACE cap screws:

The ACE screws have a flat face but a rounded edge. It seems to me that since the car had the blunt nub with the gold booster for numerous miles, the ACE screw wouldn’t pose a problem. It could be argued that the ACE nub makes more contact than the tiny edge of the newer booster nub.

 

[5.7 hemi]

Have ya looked at the fulcrum under the dash to see if there’s any “play”? Remember, most aftermarket boosters are actually meant for GM garbage unless ya got one meant for a Mopar.

Forgot to add…..if you’re using a small bore mc with a booster, pedal feel will crappy.

 

[68 Sport Satellite]

Nice work Greg. Supposedly to allow for heat expansion, I've read that ideally there should be a .020" gap between the adjusted nub and the MC bore piston face. Where's that dang H-shaped tool for this that I thought I brought but can't seem to find? Master Power Brakes has a nice write-up on this to solve the too soft pedal issue we're observing. Too close though and the pedal will be very grabby and could even make the pads drag.
Got A Soft Brake Pedal? How to check the Booster Pin to Master Cylinder Gap

 

[Kern Dog]

The dash side is factory stock stuff. It seems as proper as what is in my car.
That gauge wasn’t in the trunk. No matter. It doesn’t take long to adjust this nub to change the pedal free play.

 

[68 Sport Satellite]

Have ya looked at the fulcrum under the dash to see if there’s any “play”? Remember, most aftermarket boosters are actually meant for GM garbage unless ya got one meant for a Mopar.

Forgot to add…..if you’re using a small bore mc with a booster, pedal feel will crappy.

using 1-1/8" MC for front disc / rear drum. Dot 3 fluid, all new lines no leaks. The Booster is Bendix 8" dual diaphragm from Dr Diff.

 

[Kern Dog]

With the nub backed out and the free play almost gone, the feel is only slightly better than it was with the old booster. It is hard to explain a “feel”.
For the record, I have a 15/16” MC in my car and to me, the feel is excellent.

 

[68 Sport Satellite]

With the nub backed out and the free play almost gone, the feel is only slightly better than it was with the old booster. It is hard to explain a “feel”.
For the record, I have a 15/16” MC in my car and to me, the feel is excellent.

you mean it now feels almost like with the gold Master Power booster that was in before?

 

[Kern Dog]

Yes.
The pedal travel is short where the brakes almost feel off, then on. Not as linear as before where you press harder and they grab harder.

This is why I was hoping to get feedback from others to see if the free play is a common thing among others. Three cars here at the shop with the same booster but slightly different combinations and to me, they all had a bit of free movement before the brakes grabbed.
What is acceptable may vary from person to person but if it is the nature of the design, it may be a matter of living with it.

 

[68 Sport Satellite]

That means the change had an effect. That's good. The sweet spot is somewhere in the middle.

 

[68 Sport Satellite]

Even without the tool, can you shoot for a .020" gap and report back? Text is fine too.

 

[steve340]

I set the pushrod clearance from 0 to 5 thousands. Too much clearance and you get a low pedal due to the pedal ratio as this multiplies any clearance and the application can become "violent"
As you press harder on the brake pedal the line pressure should go up causing the brakes to apply harder.
Thats the theory anyway.

 

[MWbirdLOVER70]

This is a great tech post that I’m watching Kern/68 SS

 

[steve340]

Does the booster still have the reaction pad in place?
That could explain the lack of "feel"

 

[Kern Dog]

“Reaction Pad” ?
I don’t know what that is.

 

[steve340]

It is a firm rubber pad under the booster pushrod and between the pushrod and the diaphragm of the booster.

 

[Kern Dog]

I set the pushrod clearance from 0 to 5 thousands. Too much clearance and you get a low pedal due to the pedal ratio as this multiplies any clearance and the application can become "violent"
As you press harder on the brake pedal the line pressure should go up causing the brakes to apply harder.
Thats the theory anyway.

Years ago I adjusted the pushrod nub in my car with the same goal here: To shorten that free play. It wasn’t a “one and done” thing. I adjusted, drove it, adjusted until the brakes dragged, then backed it off. In the end it was a slight improvement but the booster itself just wasn’t adequate. It was a single diaphragm A body unit that was designed for lighter cars. It worked well with front disc/ rear drum
But the 4 wheel discs needed more pressure to work right.

 

[Kern Dog]

It is a firm rubber pad under the booster pushrod and between the pushrod and the diaphragm of the booster.

If that is inside the booster, it was untouched. This booster is barely a two months old. The pushrod stayed in the booster when I replaced the nub.
I know there is a better technical term than NUB but I don’t know what it is supposed to be called.