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Old 01-17-2013, 04:29 PM
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which booster do you have?
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  #17 (permalink)  
Old 01-18-2013, 09:51 AM
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Verify the bore of the master cylinder. My guess is that it is 1 1/8". Install a smaller bore master cylinder to increase line pressure. If all else is good, that will solve your problem. Dropping down 1/16" in bore size makes a very noticeable change in pedal feel. If it is as hard of pedal as you describe, you might consider going to a 1" bore diameter. You will increase pedal travel and increase line pressure.

Regards,

Andy
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Old 01-18-2013, 07:40 PM
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its the second one you posted, the one with the glossier finish
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Old 01-18-2013, 09:36 PM
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Thanks, how do I measure the bore size? Ive done some searching on it and none of them describe how to measure the GM bore size.
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Old 01-18-2013, 09:56 PM
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To measure the bore diameter, remove the master cylinder from the booster (often you can do this without removing the brake lines) and with a caliper measure across the hole that the master cylinder piston rides in. That is the bore of the master cylinder.

For power brake systems on hot rods, 99% of the master cylinders out there are either 1", 1 1/16" or 1 1/8". GM did make some step-bore master cylinders, but they are uncommon and not typically used on a hot rod.

Many of the commonly used master cylinders that come with the aftermarket kits have different bore diameters available so you won't have to redo your brake lines to swap it out.

Keep us posted on how you are doing.
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Old 01-19-2013, 01:31 PM
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its the second one you posted, the one with the glossier finish
That is a dual diaphragm booster. It should be more than enough. Master cylinder bore, or pedal ratio is likely your problem.
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Old 01-20-2013, 11:46 AM
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alright guys, ripped out the MC and my caliper shows it as 1 1/64. Would that be 1", 1 1/16 or 1 1/8...?
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Old 01-20-2013, 12:24 PM
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Quote:
Originally Posted by chevyrestore View Post
alright guys, ripped out the MC and my caliper shows it as 1 1/64. Would that be 1", 1 1/16 or 1 1/8...?
Might be a honed-out 1". Is it new or reconditioned?
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Old 01-20-2013, 01:17 PM
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its just a regular MC from classic performance. The caliper I used was a harbor freight caliper switched to the fraction function so I wouldnt count on it being 100% accurate, but close enough to decipher the size? I would guess its a 1" if my caliper showed 1 1/64 correct? CPP only sells 1" and 1 1/8" master cylinders.

If it is a 1" and thats what you guys recommend what should I check next? Ive messed with the adjusting rod on the front of the booster that changes the pedal height, still hard when its set to have the pedal travel more and same feel when its set to have less travel.

Last edited by chevyrestore; 01-20-2013 at 01:41 PM.
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Old 01-20-2013, 04:17 PM
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figure your brake pdal ratio http://www.mpbrakes.com/uploads/docu...alratiopdf.pdf
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Old 01-20-2013, 04:23 PM
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well judging by the fact that I can have the ratio LOW and still not be able to do a panic stop and same with a HIGH ratio wouldnt that mean the ratio is not the problem in my case? Right now I have it set pretty high and its still rock hard on a panic stop, according to the ratio diagram I should have a more sensitive feel how its currently set.
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Old 01-20-2013, 04:39 PM
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pedal ratio (close guess since the floorboard was in the way tape measure showed x=11.5" y=2.5" total 4.6 so that would be 4.6:1 correct? Looks like thats probably pretty good if I am reading it correctly
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Old 01-20-2013, 04:45 PM
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If you could post a photo of the master cylinder I would like to see the one you are using.

Measure the bore again with a tape measure and see if it is still just over an inch.

Have you checked the adjustment of the push-rod between the booster and master cylinder for proper adjustment? There should be just enough clearance so the pushrod doesn't push the master cylinder at all when it is bolted to the booster.
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Old 01-20-2013, 04:46 PM
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Just a thought....when I bought this power brake setup I was using a disc drum system, now its disc disc. All I did was remove the inline residual pressure valve, is it possible this master cylinder is incorrect for a disc disc setup?

I was told by CPP that all I needed to do was remove that wilwood inline valve that they had me put in the brake line but I dont know how much I believe that now, how can I confirm that this master cylinder will work for a disc disc setup as it sits?
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Old 01-20-2013, 04:50 PM
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this link is for the same master cylinder Classic Car and Truck Brake, Steering and Suspension Parts

It may or may not say disc disc or disc drum but when I bought it I told them disc drum and all they said was I would either use it as is for disc disc or add the inline valve for disc drum.

As for the booster, I have the adjuster nut on the booster turned all the way in and it feels like it is just barely touching the master when I bolt up the master to it. The pushrod between the master and booster is non adjustable.
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