Setting brake booster pin depth

[themechanic]

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The Benefits of Booster Pin Depth Gauges
Posted by Master Power Brakes Team on Mar 24, 2020 2:12:00 PM

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Let’s talk about booster pin depth and why it’s important. The booster pin depth, which is often overlooked when installing a new master cylinder, can drastically affect the comfort and efficiency of your driving experience. We’ll discuss symptoms of improper depth and how to properly set it.

A brake booster pin, also known as a brake push rod, works as you press the brake pedal with your foot. The booster pin extends into a piston in the master cylinder to initiate the braking process.

Booster Pin Depth
The booster pin depth should leave a small gap between the end of the pin and the master cylinder. We recommend the ideal depth of 0.020 inches.

If the booster pin depth is not properly set, you will notice several unpleasant symptoms while attempting to slow or stop your vehicle.

Brake Drag
On a manual brake system, you may experience brake drag, an uncomfortable feeling of the brake pedal not releasing as a result of the booster pin pushed up too closely against the master cylinder. Brake drag needs to be repaired as soon as possible, since it can force parts to overheat, damage the braking system, or cause an accident.

Soft Brake Pedal
If the push rod depth is set too long, there will be a delay from the time your foot presses the brake pedal until the brakes engage, causing a soft brake pedal.

SIDEBAR - An example of an unfortunate experience: Imagine a gap of 0.250” between the pushrod at the booster and the back of the master cylinder. With a gap of 0.250” and the pedal ratio 4:1, the brake pedal pad where you actually place your foot to stop will move an entire inch before anything happens. The feeling of a soft brake pedal is not comfortable while driving.
Setting the Booster Pin Depth
It’s easiest to set the depth of the booster pin when you install the master cylinder. We make this process simple with the help of our booster pin depth gauge (AC2003). While using the booster pin depth gauge tool is not a requirement, it does make the job very easy by eliminating the need to take multiple measurements and calculate the correct math.

How to Set Booster Pin Depth
1. Check the piston depth at the master cylinder.
2. Flip the tool and use it to check the depth at the booster.
3. Adjust the pin as needed to set at a depth of approximately 0.020”.
4. Assemble the master cylinder onto the brake booster.

Here is a quick video demonstrating the use of the booster pin depth gauge.



Benefits of Setting the Booster Pin Depth
Eliminate the symptoms of an improper depth setting, including brake drag and soft brake pedal, by setting the booster pin depth to a proper distance. This will make your ride experience much more comfortable, safe, and enjoyable, all while eliminating wear on your vehicle. Setting the depth is easy and accurate with a booster pin depth gauge.

 

[steve340]

I have got one of those tools and they do make the job very easy.
Well worth having in your toolbox. I have loaned mine to several people and feedback is always good.
However the spec I set them to is around 0.005"

 

[dadsbee]

 

[Mopars & Missiles]

View attachment 931815

Pretty much exactly how I do it too.

 

[Cornpatch MO]

I put power brake booster and toggle link on my 68 Plymouth witch came from the factory with non boosted brakes. I bought a rebuilt booster from my local parts house. It did not have installation instructions. I could not get the brakes to work , and finally took it to a mechanic. He told me about the pin setting , and mine from the rebuilders was way off.. I had a real hard time setting the depth, really wished I had one of those tools, but I finally got it set fairly close...Problem solved......................MO

 

[themechanic]

I bought the plastic one off Amazon since I only need it once or twice in my lifetime. You can get the fancy aluminum one for $$ more.

 

[Ron H]

Was looking around for this gauge...even all the mechanic buddy's didn't have one of them. I've seen the better ones for around 30 bucks though yeah, plastic or nylon ones are less than half the cost. I've posted before about problems I've had with my front disk to power conversion just posted on another this morning seeing the guy has the identical problem I'm having. I have mysterious pedal sink but does not cause the brakes to engage. While pedal is way down, fortunately I still can push it further and brakes work fine. There's a little force required, a resistance, to pull pedal back to its return than it works fine...until the next time it happens. Waiting to see if anymore posts show up. Several have posed about a bad booster and MC though I've done the recommended vacuum checks on the booster and checks out fine. I installed a reserve vac tank since I have a mild cam and maintain 15" of vac cruising. That didn't make any difference. Now I read that there could be something about pedal 'ratio' I know nothing about and also pedal free play I need to explore. I wonder if the pedal travel 'return' is not complete if this could be an issue? I figure next check I'll move the brake light switch out of the way to see if further release might happen. What I'm saying is if the booster isn't fully released if this could cause pedal sink?

 

[steve340]

RonH
I really think your problem is a bypassing master or rear adjustment/push rod or something on the hydraulic side of the brakes. The booster is only a mechanical advantage device - in short it helps you push the pedal it will not make the pedal spongy or pump up.

 

[Ron H]

RonH
I really think your problem is a bypassing master or rear adjustment/push rod or something on the hydraulic side of the brakes. The booster is only a mechanical advantage device - in short it helps you push the pedal it will not make the pedal spongy or pump up.

If I'm following ya, the rear adj/push rod you mention has to do with the booster...it might not completely be disengaging due to the pedal rod adjustment maybe...such as not enough return travel?

 

[steve340]

No if anything the booster push rod has too much clearance. If you did not have enough clearance when the master returned it would not uncover the compensating ports allowing fluid to return to the reservoirs. This would lock the brakes on eventually.
By rear I meant your rear drum brakes. They could be out of adjustment - meaning you have to pump them out, the brakes then feel OK but if you do not use them for a minute or two the brake shoes draw back.
A way to test is get the car on jack stands and wind up the rear adjusters to lock the brake drums. Back the emergency brake adjustment off before you do this and do not use this as a cheats way to lock the drums for the test. Always adjust the wheels adjusters first then do the emergency brake adjustment. Check your pedal - if the problem is gone or changed you know where to look.

 

[steve340]

This is a video you may not have seen but may help you understand your problem.
It is on YouTube
Chrysler Master Tech - 1970
Volume 70-9 Brake Hydraulics

 

[CoronetDarter]

When I first converted to power disc I didn't know about pushrod depth. Turns out the m/c was partially engaged all the time - explained why the rear drums locked up just touching the pedal.
Used the caliper and straightedge method like dadsbee to finally get it right.

 

[Kern Dog]

I just did it by adjusting, driving the car, adjusting...until it was right.
FREE.

 

[myk r sanchez]

So you adjust the pin by rotating it in the master cylinder?

 

[steve340]

No it is a threaded adjuster on the end of the brake booster push rod.

 

[dadsbee]

I just did it by adjusting, driving the car, adjusting...until it was right.
FREE.

You take the master on and off repeatedly until you get it working right? Interesting...

 

[Kern Dog]

Four nuts, pull away about 3 inches, adjust, test....Easy.

 

[Lineman]

This thread is good information, but is it my problem? I have a 69 GTX with factory disks. Somebody had a Summit booster on it so I assume it was a power brake car. I just put the almost correct booster on it from inline tube with a new master cyl. The brakes are pretty touchy and it seems to be stopping on the rear brakes mostly. I am going to say 75%.. on the rear brakes maybe more. The rears are getting pretty hot when I check them and you can put your hand on the fronts.. I think when you really mash the brakes it would turn the car around quick.. I have pulled the master cyl and it doesn't seem like the rod is pushing on the piston in the master cyl..

Can anybody put me on this one..

Thanks

Jay

 

[Mopars & Missiles]

This thread is good information, but is it my problem? I have a 69 GTX with factory disks. Somebody had a Summit booster on it so I assume it was a power brake car. I just put the almost correct booster on it from inline tube with a new master cyl. The brakes are pretty touchy and it seems to be stopping on the rear brakes mostly. I am going to say 75%.. on the rear brakes maybe more. The rears are getting pretty hot when I check them and you can put your hand on the fronts.. I think when you really mash the brakes it would turn the car around quick.. I have pulled the master cyl and it doesn't seem like the rod is pushing on the piston in the master cyl..

Can anybody put me on this one..

Thanks

Jay

Previous posts have shown you how to set the push rod (match it) to your Master Cyl., so be sure to check that. FSM shows the spec and how to do it. IIRC, the push rod length should be about 0.900" +/- 0.020. Check the manual and measure your M/C depth and push rod extension before mating the two together.

Front disk/rear drum car needs to have a proportioning valve to set the front/rear brake ratio. Since you say your car is a "factory front disk" car, it should have a factory proportioning valve on it unless it was removed at some point. If not, you need to get an aftermarket proportioning valve and install it in the rear brake line.

What you are describing as the rears doing 75% of the work is very DANGEROUS. In a panic stop situation, the rears will LOCK and you will most likely NOT be able to control the car, as the back end will try to come around in front of you. In other words, you WILL spin.

One other thing, there is a big difference between a "proportioning valve" and a "distribution block". Many people confuse the two.

 

[Ron H]

Among the early checks would be to adjust the proportioning valve - it could be the adjustment has most pressure going to the rears. Without a line pressure gauge the trick is adjusting it and taking it for a drive to check the braking action. Preferably on a gravel road. If you have a booster rod gauge, this would be one way, easiest method, to check the gap between the booster and MC. Gaps vary; but I adjsuted mine to around .10 gap.