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Brake booster doesn’t fit master cylinder

bentendo

Full Member
Joined
May 16, 2010
Messages
232
Got a new booster to cure a vacuum leak. It’s from Duff’s, where I got my power brake kit originally. The posts on the booster don’t match the mounting holes on the master cylinder, they’re too narrow spaced. Is drilling out one of the holes an acceptable solution?

891d20c2aed2d7c5748bb40d37030b20.jpg



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sykanr0ng

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Aug 11, 2014
Messages
5,363
The first thing I would do is contact Duff's about that, the parts they sell should fit the other parts they sell.

If you decide to enlarge the holes I would file the inside of both holes so that the mounting is even instead of shifting it to one side.
A chainsaw file would be my first choice, a rat tail file if you can't find a chainsaw file.
 

DirtDonk

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x2
Opening up the holes is an acceptable practice (and some masters actually now come with oval holes for this reason) but like said, give Duff a call to see if they know what's up.
They can confirm the measurements with the parts in stock to make sure yours is the correct unit first and foremost.
They may also know if their older kits had parts of a different size. That's very possible because for awhile it was hard to get legit Bronco boosters. Maybe their kit at the time was meant to overcome that obstacle.

Good luck no matter which door is the winner!

Paul
 
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OP
bentendo

bentendo

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May 16, 2010
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Called, it’s an entirely different MC, which I’ve ordered and will have in ASAP. Thanks for the input!


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DirtDonk

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Well at least you won't have to worry about whether the old one is going to last any longer or not!

Make sure to verify the rod adjustment before you button things up too.
You're familiar with what all that involves? It's simple, but very important to verify the rod does not touch the piston before the master is seated against the booster.
But you don't want too much gap either, or your pedal will travel further before you get full braking.

Everything is simple, right? They all just take more than one step to get there!;);D

Paul
 
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bentendo

bentendo

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May 16, 2010
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232
I knew there was an adjustment nut on the push rod, but how do you verify it’s not touching? Measure the distance from mounting surface to plunger on MC, and then mounting surface to end of pushrod on Booster and set a 1/16 gap or so?


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DirtDonk

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Yes, that's exactly how you're supposed to check. But there is the "manual" way as well.
And that's to adjust the rod out to the point that it bottoms out on the back of the piston just before the master cylinder mounting flange touches the booster face, then you adjust the rod back in to the point that you can mate the two items together before the piston touches the rod.

A 1/16" gap is too big. Should be less than half of that if I remember.
I used to use 1/64" as my guide, but there is an official specification in the book and someone posted it up here last year so it should be around.
I'll see if I can find what Chilton's or Haynes lists, but as long as you don't have the rod touching the piston you're gettin' close.

The tight gap is at least partly because the pedal ratio could be at least 6:1 which means that a 1/8" gap would equate to an extra 3/4" of pedal travel before the piston even starts to move.
An 8:1 pedal ratio would give you a full inch at the pedal before the initial brake travel starts. Too much gap and you could end up with perfectly good stopping brakes, but a pedal that's almost at the floor before you stop!

So less is better, up to a point.
I'm sure you'll find the exact spec. And you can use the touchy-feely method I described and get it pretty close. But the measure-it-with-the-proper-tools method is what gets you right on the money as far as the factory spec is concerned.

Paul
 

PDQ

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May 6, 2012
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Bent, Did the new master work out? I must have the same set up from the same vendor, and the booster sent to me not only doesn't fit the master, but now doesn't clear the fenderwell either. So I'm afraid if they just send a new master to me, then I'll still have the new fenderwell issue. Wondering if your issue was solved with the new master.

Thanks
 
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bentendo

bentendo

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May 16, 2010
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Yeah, it all fits with my setup. The kit was bought in… 2012? Sometime around then.


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