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#11543 05/14/2005 12:43 AM | Joined: Feb 2000 Posts: 364 Shop Shark | | Shop Shark Joined: Feb 2000 Posts: 364 | My '49 Burb has 4 wheel disc brakes with the master cylinder mounted on the drivers side frame under the cab.
I have to pump the brakes at least once to get a decent pedal. We've pressure bled the system twice...there are no leaks and there is plenty of fluid in the system.
I've recently read that you need to install a 2lb "residual" valve in the system when the calipers are higher up than the master cylinder. Is this true? and if so where does this valve go? | | |
#11545 05/14/2005 1:10 AM | Joined: Feb 2000 Posts: 364 Shop Shark | | Shop Shark Joined: Feb 2000 Posts: 364 | Thanks for the help...a couple of add'l questions; why do you need them, what do they do? | | |
#11548 05/14/2005 5:19 AM | Joined: Feb 2004 Posts: 29,742 Kettle Custodian (pot stirrer) | | Kettle Custodian (pot stirrer) Joined: Feb 2004 Posts: 29,742 | I've always heard that residual pressure valves will cause disc brakes to drag, because there's no spring pressure to retract the pads, like a drum brake has. At least, when one of my friends used a master cylinder with anti-bleedback valves on his race car, all 4 disc brakes had enough drag to turn the rotors red hot. When we changed to a MC without the residual-pressure valves, the drag stopped happening.
It sounds like you need a MC with a bigger bore size, to displace more fluid on the first stroke.
Jerry
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#11549 05/16/2005 5:25 AM | Joined: Oct 2001 Posts: 3,458 Extreme Gabster | | Extreme Gabster Joined: Oct 2001 Posts: 3,458 | Disc brakes are supposed to drag against the disc. The reason you have to pump is that they are draining back and retracting too far. Disc brakes should have a 2lb residual valve if the MC is below the wheels. Drums should have a 5 lb residual valve. If you put a drum residual valve on a disc system you will get the same problem as hot rod lincoln.
An MC higher than the wheels solves this with gravity.
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