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Old 06-26-2007, 07:40 PM
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mc/power booster

I need help on this one. How can I determine the compatibility of my mc and power booster???????????????? I have a lunati cam in a 383 sbc, 7" dual diaphram booster, trying to stop a 3900# 1960 Chevy pu truck with '81 camaro/fire bird single piston OEM calipers. Brake pedal sucks. Is there anyone that can advise me??

: confused:

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Old 06-27-2007, 06:30 AM
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You generally need no less than 18" of vacuum at an idle for good booster performance. They will work with less vacuum than that, but the boost effect will be less, and the pedal will be hard. There are electric vacuum pumps that will make the proper amount of vacuum to run a brake booster.
How much vacuum do you have at an idle? at cruise?

Your system description is somewhat vague, writing "brake pedal sucks" doesn't tell us if it is high and hard, low and soft, just doesn't stop the truck or what.

Usually those systems work with a 1" or 1 1/8" bore master and a pedal ratio in the range of 4 or 5 to 1, but alot depends on your rear brakes as well.


Masterpower brake has a really good page on sorting out brakes.

http://www.mpbrakes.com/technical-support/reference.cfm

So does CPP


http://www.classicperform.com/TechBo...hoot.htm#pedal

If you can narrow down the symptoms of what your system is NOT doing , it will be easier to identify which part needs attention.

Hope this helps,
Mikey
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Old 06-27-2007, 05:36 PM
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Mikey thanks for the reply,

I need to check my vacuum. However how the brakes react is like this. The pedal is soft and mushy. My rear brakes are single piston calipers also. I probably get more braking out of the 4:88 then the brakes. It does stop but I would like a bit more brake before the pedal feels like it's at the floor. I do have a proportioning valve, but can't tell how much I have it adjusted for the rear. My rear tires are 33 X 21.5 X 15 on 15 X 15 centerline wheels. With that much tire how much brake should I have on the rear????? Unfortunately I bought the master from a different place then where I bought the booster, an I making any helpful points yet????? I mean enough info do diagnose. My 383 sbc has 11:1 c/r, the cam is a Lunati with intake lift of .485 and ex .507 duration at .05 is (actual) 231 intake and 239 ex. advertised at 288 and 296 respectively and there is 108 degrees of lobe separation. Is that to much information or even what you need to know to answer my question???? Oh ya my front wheels are also 15".


Dennis
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Old 06-27-2007, 10:26 PM
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It doesn't sound like you have so low vacuum that the booster won't work.

Most likely cause of low mushy pedal: You have air in your system.

2nd most likely cause: Your pedal ratio is incorrect for a power brake system.

3rd most likely cause. Your master cylinder bore is too small.

You may also have an improperly adjusted pushrod where it enters the master cylinder. There needs to be no more than .030" clearance between the bottom of the bore in the primary piston and the end of the rod that protrudes from the power booster.

It sounds like you are doing some type of pro street truck.
You never said what type of calipers are on the rear, but the mid size metric GM calipers that you put on the front are a bit small for a 3900 pound truck. Are they on the 11 rotors? If you are running short skinny tires they'll probably be adequate.

IMO, the earlier midsize car calipers would have been a better choice as they make more pressure at the pad and are a little stiffer.

HTH, mikey
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Old 06-28-2007, 10:03 AM
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powerrodsmike has given you very good information. If that doesn't work check out this info.

What master cylinder are you using?

If it is for a disc/drum setup you will get poor rear braking, spongy pedal and it will feel like your pedal is going through the floor. The disc/drum master will not supply enough fluid and pressure to your rear discs as the piston does not have a long enough stroke.

The single piston calipers should do the job as they were used on everything with disc brakes for 30+ years. The 11 inch rotor may be a bit small for your heavy truck and if that is the problem you would get better braking if they were 12 inch.

If it is for a disc/disc setup then you will want it to be a 1 1/8 inch bore to reduce pedal travel.

Where is your master cylinder located?

If is located below the calipers (frame?) you will need a 2 pound residual valve in both lines to prevent the brake fluid from flowing back to the master or you will get that spongy pedal effect.

I am not sure why you have a proportioning valve but because you do you should also have a metering valve in the front brake line. It will hold off the pressure to the front brakes until the rear brakes start to work and then release pressure to the front.

Hope this helps as driving with those problems is not a safe situation. We want you to be around and share your experiences with us on the site and not from a hospital bed.
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Old 07-16-2007, 07:03 PM
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OK guys, I'm eternally grateful for all your help. This may take a bit so here we go. We bled the brakes starting with wheel farthest from the master and worked our way forward, found a bit of air and thought that was it. Put 'er down on all fours and went for a spin, did't take till the end of my driveway to notice a slight bit of a change. Went back checked the vacuum about 20 vacuum not a problem. Took off the booster measured the bore of the master 1 1/8" and low and behold dumb *$# me never adjusted the booster rod to the master, it was a good 1/4" back, I then set that about .030 away, put it together, adjusted the brake pedal in the cab and OH my brakes about 1" from the top. Guys thanks for steering me the right direction and saving me tons of bucks. The web site you had me go to (http://www.classicperform.com/TechB...shoot.htm#pedal) was great along with your advice. I'll soon stick a pic of my truck on the forum.

Dennis
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Old 07-17-2007, 11:35 PM
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Glad to hear all things worked out for you!
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