Pushrod Clearance On Brake Booster

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Psucamper
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Pushrod Clearance On Brake Booster

Post by Psucamper » Tue Feb 11, 2014 10:05 am

After installing a new master cylinder (79 Westy) I am no longer sure that the proper clearance for the compensating ports is correct. Read the Bentley info but the fine details aren't there. I know about the two "clicks" one is supposed to hear but are they really audible and from where? If I rotate the connecting rod to change its length can I actually feel the desired 0.040" clearance? Can someone suggest a step-by-step procedure? Thanks

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hambone
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Re: Pushrod Clearance On Brake Booster

Post by hambone » Tue Feb 11, 2014 1:20 pm

I just faked it. You can hear/feel when the brake pedal engages with the master. You just want a hair more "loose", just before the rod touches.
I always fly by the seat of my pants anyway...
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Psucamper
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Re: Pushrod Clearance On Brake Booster

Post by Psucamper » Tue Feb 11, 2014 3:48 pm

Hambone...Thanks for the reply. I've done a lot of Bug master cylinder replacements and setting the push rod clearance is pretty easy. The "feel/click' as the push rod strikes the MC is easily found. From your description it seems like the busses are similar but the "dip" at the pedal may not be the same as a sedan given the added complexity of the different hardware and booster. Don't know. I'll try and see.

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hambone
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Re: Pushrod Clearance On Brake Booster

Post by hambone » Tue Feb 11, 2014 5:50 pm

It's pretty much the same gizmo, but with the booster "in the way".
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kreemoweet
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Re: Pushrod Clearance On Brake Booster

Post by kreemoweet » Tue Feb 11, 2014 10:27 pm

The 1 mm (or thereabouts) clearance at the brake pedal linkage/booster interface is easy to see and feel, and it is not at all critical. The clearance between the actual push rod inside the booster and the m/c piston is something else. I don't think there is any direct mechanical connection between the brake linkage and the pushrod.

There are cases reported where the push rod was too long, and it gave no indication other than brakes that would
not release properly. To check, you can unscrew the brakelines, remove the o-ring from the flange of the m/c, and place it in position on the booster. You should be able
to feel if the pushrod is hitting the piston (bad). Some booster pushrods have screw-on tips with a washer underneath, and changing the washer can alter the length of the
pushrod.

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Amskeptic
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Re: Pushrod Clearance On Brake Booster

Post by Amskeptic » Wed Feb 12, 2014 9:01 pm

Psucamper wrote:Hambone...Thanks for the reply. I've done a lot of Bug master cylinder replacements and setting the push rod clearance is pretty easy. The "feel/click' as the push rod strikes the MC is easily found. From your description it seems like the busses are similar but the "dip" at the pedal may not be the same as a sedan given the added complexity of the different hardware and booster. Don't know. I'll try and see.
Go under the bus and pull down on the pedal arm just enough to see that you have taken away any clevis pin play. The next discrete movement will be the actual pushrod to the booster. You will feel it hit the control valve in the booster, then the master cylinder piston. This is the click you need. It is easy to loosen the locknut and turn the threaded push rod to get your 1mm clearance.
Colin
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Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
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Psucamper
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Re: Pushrod Clearance On Brake Booster

Post by Psucamper » Thu Feb 13, 2014 5:41 am

Thanks to all who responded to my inquiry about booster push rod clearances. It appears that it's not as difficult as first thought. There may be a slight delay in accessing the underside of the bus as the garage temperature is currently a pleasant +5 F. Persistent...unrelenting......cold....snow....ice.

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wdollie6
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Re: Pushrod Clearance On Brake Booster

Post by wdollie6 » Thu Jul 28, 2016 7:35 am

Okay, so as I begin to button up my 72 Westy, I'm a little confused on the pushrod clearance issue. In Colin's last reply to this thread he indicated that you could essentially adjust the 1 mm clearance by adjusting the threaded pushrod on the booster, can it be done from there or does it have to be accomplished by pulling the master cylinder off of the booster and adding or removing spacers? I have correctly adjusted the brake stop per Bentley now just want to make sure that the 1 mm clearance is correct. Yes, I already bled all the brakes successfully (I know should have done this adjustment first) and the brakes appear to work properly, however want to verify that all is correct to avoid problems later.

Thanks in advance.
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kreemoweet
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Re: Pushrod Clearance On Brake Booster

Post by kreemoweet » Thu Jul 28, 2016 8:50 pm

It's confusing, yes, because there are 3 different things being called "pushrod" in this discussion. With regard to boosted bay bus brakes, that term
usually refers to the brake pedal linkage piece that interfaces with the booster. That is the only thing normally adjustable, and nothing you do there
will correct any bad clearances in the other "pushrods". If the rod in the booster that actually contacts the master cylinder piston has an incorrect
clearance, then sometimes, depending on the make of the booster, that can be corrected on the booster part.

There is NO "1 mm" clearances specced anywhere for any of the bay powered brake parts, but it's not a bad target to shoot for in both the pedal-linkage-to-booster gap, and the internal pushrod-to-M/C-piston gap.

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Amskeptic
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Re: Pushrod Clearance On Brake Booster

Post by Amskeptic » Mon Aug 01, 2016 10:07 am

Ignore all "clearances" but the internal booster-to-master cylinder clearance. This is predicated on the correct o-ring between the master cylinder and the booster with the master cylinder correctly torqued to the booster. Your adjustable push rod can and will be "preloaded" against the booster spring a little bit. You can feel the booster push rod click against the master cylinder after you have adjusted out the slop at the clevis.
hth Colin
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

72Hardtop
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Re: Pushrod Clearance On Brake Booster

Post by 72Hardtop » Sat Nov 17, 2018 12:04 pm

If one looks closely at the end of the push rod on the ATE booster you'll see that it has a small washer (~1mm). The tip can be removed (pulled off w/vise grips) if one needs the clearance. Best to look at the old booster push-rod after removal and compare it to the new one. Then decide if that is needed/required.

Shot of the end of an ATE booster rod...

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Amskeptic
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Re: Pushrod Clearance On Brake Booster

Post by Amskeptic » Sat Nov 17, 2018 5:48 pm

kreemoweet wrote:
Thu Jul 28, 2016 8:50 pm
It's confusing, yes, because there are 3 different things being called "pushrod" in this discussion.

There is NO "1 mm" clearances specced anywhere for any of the bay powered brake parts,
See: Brakes 4.1 fig 4.1 and note that "on 1971 and later models, which are equipped with front disc brakes, it is very important that the correct clearance exist ... between the push rod and the master cylinder piston." Fig 4-1 1mm/.040"
BobD - 78 Bus . . . 112,730 miles
Chloe - 70 bus . . . 217,593 miles
Naranja - 77 Westy . . . 142,970 miles
Pluck - 1973 Squareback . . . . . . 55,600 miles
Alexus - 91 Lexus LS400 . . . 96,675 miles

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