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| | | | Joined: Apr 2005 Posts: 2,832 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 2,832 | They have no spring and the mc cap is vented. My 34 Chevy has discs on all four corners, everything new and near zero run out so figured 2# residual pressure valves not needed--wrong. My porta power ram draws back but figured reservoir is not vented so vacuum is formed. Conversely the ram on my big press needs two strong springs to retract it. What's happening?
Evan
| | | | | Joined: Sep 2010 Posts: 1,706 back yard wrench turner | | back yard wrench turner Joined: Sep 2010 Posts: 1,706 | Where is your M/C located? If under the floor, gravity will draw the fluid back. This is why the residual valves are needed.
Wayne1938 1-Ton Farm TruckStovebolt Gallery Forums1989 Silverado 1996 Mustang GT convertible When I die, I hope she doesn't sell everything for what I told her I paid for it! | | | | | Joined: Jan 2009 Posts: 1,644 Shop Shark | | Shop Shark Joined: Jan 2009 Posts: 1,644 | In You're porta power there is a spring on the inside of the ram.Disc brakes don't really retract they just quit squeezing on the rotor when you let you're foot off the pedal.Disc brakes don't need residual valves gravity is not going to force the caliper to retract.
Pete | | | | | Joined: Sep 2010 Posts: 1,706 back yard wrench turner | | back yard wrench turner Joined: Sep 2010 Posts: 1,706 | There is a spring in the M/C that will also draw fluid back.
Last edited by Wayne67vert; 02/25/2016 1:55 PM.
Wayne1938 1-Ton Farm TruckStovebolt Gallery Forums1989 Silverado 1996 Mustang GT convertible When I die, I hope she doesn't sell everything for what I told her I paid for it! | | | | | Joined: Oct 2006 Posts: 9,671 'Bolter | | 'Bolter Joined: Oct 2006 Posts: 9,671 | When there is no hydraulic pressure on the pad it is not pushing
on the disc so there is very little friction between the two.
The disc cools down quite rapidly when the friction is reduced
therefore it shrinks a few thousands creating a clearance.
dg
Denny G
Sandwich, IL
| | | | | Joined: Jun 2004 Posts: 8,597 Riding in the Passing Lane | | Riding in the Passing Lane Joined: Jun 2004 Posts: 8,597 | The seal ring inside the caliper is at a slight angle. The lip of the seal pulls the piston back just enough so it doesn't drag the lining.
They say money can't buy happiness. It can buy old Chevy trucks though. Same thing. 1972 Chevy c10 Cheyenne SuperIn the Gallery Forum | | | | | Joined: May 2005 Posts: 8,878 . | | . Joined: May 2005 Posts: 8,878 | If the disc is not running true that could also bump the piston back a tad. | | | | | Joined: Feb 2000 Posts: 5,215 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 5,215 | All true, but the seal in the caliper does most of it. They are square cut on a slight angle, the fluid pushes the piston and seal, when released, the seal pulls back into shape bringing the piston with it. I have been running disk/drum set up since 1999 with no hold back valves, didn't know I needed them. Pedal is stiff each time i need it and master cylinder is the original location.
| | | | | Joined: Jun 2009 Posts: 1,596 'Bolter | | 'Bolter Joined: Jun 2009 Posts: 1,596 | In You're porta power there is a spring on the inside of the ram.Disc brakes don't really retract they just quit squeezing on the rotor when you let you're foot off the pedal.Disc brakes don't need residual valves gravity is not going to force the caliper to retract.
Pete Not true. Depending on the master cylinder location, some disc brake setups need a 2PSI residual valve. | | | | | Joined: Apr 2005 Posts: 2,832 'Bolter | | 'Bolter Joined: Apr 2005 Posts: 2,832 | Thanks everyone,
Master cylinder is frame mounted and pedal gets hard one inch from floor. Pump once and has half pedal; twice and full pedal. Pedal stays hard in each location. With new rotors, bearings and spindles I get less than 0.001 run out so knew something had to be retracting the pads.
Evan
| | | | | Joined: Jun 2009 Posts: 1,596 'Bolter | | 'Bolter Joined: Jun 2009 Posts: 1,596 | Thanks everyone,
Master cylinder is frame mounted and pedal gets hard one inch from floor. Pump once and has half pedal; twice and full pedal. Pedal stays hard in each location. With new rotors, bearings and spindles I get less than 0.001 run out so knew something had to be retracting the pads. It's common to need residual valves with a frame mounted master cylinder. | | |
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