Free play in power brake systems.

[steve340]

I got what you meant no worries.
The reaction disc is sometimes lost/displaced and winds up in the booster case.
I goggled "brake booster reaction disc purpose" and all the info is there.

I purchased one of the tools posted by someone earlier and they are worth their weight in gold IMO.

 

[Kern Dog]

I could see the value for some instances. If I were troubleshooting and needed answers, that would surely be helpful. It would give clear numbers to work with instead of the slower trial fitting and back and forth.
It is a funny thing… I often happen upon some great time saving tool long after I’ve found ways to do the job without one. Sometimes I jump in with the new stuff, sometimes I’m a dinosaur and stick to the old stuff.
Ratchet wrenches was one of those things that looked like some cheesy Harbor Freight BS but once I tried one, I saw the benefits of them.

 

[steve340]

I know what you mean regarding ratchet wrenches. I never had them in my toolbox and a friend I rebuilt an engine for gave me a set as a gift. Bloody love them now.

Sometimes I get to fit mismatched masters on to boosters - the tool takes all the guesswork out of adjusting the pushrod clearance. Another tool I cannot do without now and they are reasonably cheap.

 

[1 Wild R/T]

As was mentioned grabby is usually because the pushrod is to long... When the pushrod is to long the fluid can't flow back into the reservoir & the brakes can't fully release... Close but not fully...

 

[66 Sat]

Mine has an amount of "free play" that I can't get rid of...but the brakes work great, nice and progressive once you're through the dead space and I'm used to it.
This is aftermarket disc front/factory drums rear, using a distribution block but not an adjustable proportioning valve. I've had 2 boosters - the gold cheap one in the first photo that crapped out after 3 years and a reconditioned booster from PBR, a decent Australian company that provides OE brakes to many major manufacturers (the black one in the second photo.
The brakes have the same amount of free play with the rebuilt booster as they did with the original cheap gold booster. I've adjusted the pushrod using the correct tool which improved things slightly but it's still there. It's on my list to look into.

 

[Kern Dog]

That Poly mill looks very clean.

 

[CoronetDarter]

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
View attachment 1557148

I used a straight edge and the depth gauge on my calipers to measure the gap. When I first restored the Coronet I didn't know about the gap and installed the new booster and m/c without checking. Pushrod was way too long; my brakes were overheating to the point that the lug nuts on the rear wheels were too hot to touch.
After measuring and reinstalling, I think there's a little too much gap now.
I drove your car on Sunday and I didn't notice any flat spot. By comparison, my brakes feel mushy compared to yours, to the point that I need to double check the gap and possibly tighten it up a tad.

 

[CoronetDarter]

I pulled the MC forward and tried backing out the nub on the pushrod.

View attachment 1557070

I'm curious how there's that much play in the brake lines that the m/c can be pulled that far forward without kinking the lines.

 

[1 Wild R/T]

See those loops? They allow the M/C some freedom to move...

 

[Kern Dog]

The lines in my car were made by me to fit when I had the A body booster. The A unit stood off the firewall and up a couple inches so when I swapped to the stock B booster, the lines were already longer than Ideal.
With Dwayne's car, it has "loops" similar to what is visible in the photo above.

They are below the MC almost to the proportioning valve. Here you can see how far out the pushrod nub was on the first booster.

 

[CoronetDarter]

Good to know. I have the same loops in the Coronet.

 

[1STMP]

The loops in the lines are there for flex between the body and chassis
and how the brake Ines are routed and attached.
I might offer to suggest you try a 2lb
residual check valve in the front circuit.
The delay you're feeling in the pedal
may because the pads are backing
off the rotor just enough where the
fluid has to travel more before the
pistons react. The 2lb pressure is not
cumulative, so there's no worry about
boiling the fluid. It takes an average
of 5± lbs constant contact to boil.

 

[5.7 hemi]

Those are only needed if the mc is mounted lower than the brakes.

 

[ckessel]

I experienced some pushrod fun years ago when working on an AMC Scrambler. The car owner had sourced a "correct" cylinder for the car. Had issues with it applying pressure with the brake off. Turned out that the back bore in the piston was not the same as what I took out. What Rich mentioned about straight edge and depth gauge, dial caliper for me, was how I got it right. How deep was the piston hole, how far did the back end of the master stick out, how far did the rod poke out from the booster. Did the math, adjusted the rod, good to go. Unfortunately on that car, I had to remove the hood to access that area. No FFN room to work. Kind of like doing plugs on a 390/428 in a Mustang. PITA!

 

[68 Sport Satellite]

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

I had never heard of a reaction pad, so I did some reading. In many forums, drivers talk about modifying their reaction pads for improved brake pedal feel and modulation. Might be just what I'm looking for? Here's a good diagram. I know that the Bendix from Dr Diff is mirroring what was offered stock, but nothing on my car is stock. I'm going for as much improvement in steering and braking as possible. I'm curious to mess with this if possible.
KD - for research purposes, I'd be keen to cut open an old spare booster.

 

[1STMP]

Those are only needed if the mc is mounted lower than the brakes.

As a general rule of thumb, yes I agree.
However, there are instances where
a residual check valve will help with
pedal delay as it did on mine.

 

[steve340]

I am not sure if replacing the reaction pad with say a solid chunk of plastic would gain you anything substantial in the way of braking power - unless the reaction pad is damaged by say oil contamination.
On a street car - an old boss told me the reaction pad is also there to allow a little bit "give" on initial brake apply. If you take it out the brakes may become to harsh as they apply or light switch like either ON or OFF.
If you do it I would be interested in an update.

 

[Kern Dog]

The brakes felt normal to me before. There was a small amount of free play before the braked grabbed, an amount similar to the other two B body cars here at the house. The owner was used to his late model car where the pedal feel has evolved over 55 years. Driving an old car often means accepting some of the differences that will exist between them. I extended the length of the pushrod to shorten the free play and in doing so, the brakes feel as if they go from off to ON in a shorter stroke. They don't feel like they are grabbing in a linear fashion but more like a switch. To me, this is not comfortable or confidence inspiring.

 

[68 Sport Satellite]

The brakes felt normal to me before. There was a small amount of free play before the braked grabbed, an amount similar to the other two B body cars here at the house. The owner was used to his late model car where the pedal feel has evolved over 55 years. Driving an old car often means accepting some of the differences that will exist between them. I extended the length of the pushrod to shorten the free play and in doing so, the brakes feel as if they go from off to ON in a shorter stroke. They don't feel like they are grabbing in a linear fashion but more like a switch. To me, this is not comfortable or confidence inspiring.

I haven't driven it since you lengthened the rod position, but we'll just need to shorten it back up a little and drive it again. We should be shooting for a .020" gap.

Out of curiosity - I'd like to check just inside the booster opening to see if the reaction pad is there. Here's an example photo from another thread with their comment.

 

[68 Sport Satellite]

If the black rubber reaction disc is not visible, others have stated that 9 times out of 10, it fell inside the booster when removing the MC and pushrod...shake the booster and it may be rattling around inside. But my guess in this case is that the reaction disc is still there and the pushrod just needs more adjusting. The firewall side of the pushrod pushes directly against the rubber reaction disc inside the booster.