Pistons in calipers retract as far as they extend - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-17-2012, 09:25 AM
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Pistons in calipers retract as far as they extend

One of my friends called last night regarding his 71 Nova with a 4 wheel disc conversion. He's been through three new master cylinders, all the brake components are new and this one is baffling. He has bled the brakes thoroughly and properly...it isn't his first rodeo. This dang thing though requires a couple pumps of the pedal to get a firm pedal as if it has air in the line somewhere. He had somebody else pump the brakes as he watched them and the pistons in the calipers move out as they should but when the pedal is released, they retract just as far as they extended. He has a four wheel disc master cylinder mounted on the firewall same as factory, all GM style calipers and the bleeders are on the top side of the calipers. Any idea as to what might be causing the pistons to retract as far as they extend? The caliper brackets are not putting any tension on the rotor to cause a retraction either. We're both kind of dumbfounded.

Thanks in advance for any ideas,
Chip.

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Old 04-17-2012, 06:37 PM
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On new calipers, the piston dust seal often has enough tension to pull the [iston back like you discribe, particularly on turned rotors. A 2 psi residual valve usually cures it, and causes no problems on down the line.
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Old 04-17-2012, 07:11 PM
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As I understand it, some GM calipers were of a "low drag" design, and were engineered to retract the piston further back into the caliper. Mid-80's I think, so it takes more fluid volume to extend them out far enough to produce braking action. Maybe that could account for the problem he's having.
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Old 04-17-2012, 07:26 PM
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Also the master cylinder bore diameter could be too small, hence not displacing enough fluid to get adequate action at the calipers without a second pump.
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Old 04-17-2012, 07:59 PM
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As stated by Oldguy48, you probably have the low drag calipers, which were installed on quite a few eighties era GM midsized cars..Those were intended to be used with a step bore master cylinder, (like a 2 stage master cylinder, the big bore takes up the slop, and the small bore makes pressure) ALL of the low drag calipers were the metric type, but not all of the metric type were low drag. The difference is in the piston seal, the standard seal is square cut, while the low drag caliper has a slight angle cut into it, which retracts the piston after a brake application.. The idea was that without the pads toching the rotors at all, they'd save gas..
Here's a post I made a while back that describes a definitive test for low drag calipers, as well as a couple of remedies.
Test for low drag calipers

I found that installing a 2# residual pressure valve in the front brake line won't do a thing, but you can install a 10# and it will help.. A better solution is to get a set of 78-81 Malibu calipers, as they were NOT low drag.

Later, mikey
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Old 04-17-2012, 08:22 PM
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Quote:
Originally Posted by powerrodsmike
As stated by Oldguy48, you probably have the low drag calipers, which were installed on quite a few eighties era GM midsized cars..Those were intended to be used with a step bore master cylinder, (like a 2 stage master cylinder, the big bore takes up the slop, and the small bore makes pressure) ALL of the low drag calipers were the metric type, but not all of the metric type were low drag. The difference is in the piston seal, the standard seal is square cut, while the low drag caliper has a slight angle cut into it, which retracts the piston after a brake application.. The idea was that without the pads toching the rotors at all, they'd save gas..
Here's a post I made a while back that describes a definitive test for low drag calipers, as well as a couple of remedies.
Test for low drag calipers

I found that installing a 2# residual pressure valve in the front brake line won't do a thing, but you can install a 10# and it will help.. A better solution is to get a set of 78-81 Malibu calipers, as they were NOT low drag.

Later, mikey
Hey Mikey, good to see you around again been awhile.



Cole
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Old 04-18-2012, 07:55 AM
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Thanks you all! I forwarded the responses to him. This is a mishmash of parts the guy at the parts store sold him and said is exactly what he used on his and it works perfect. I'm calling shenanigans on it working perfect for the parts guy. You all did learn me on several things to do with the master cylinders and the calipers.
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