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Old 06-03-2020, 06:48 AM   #1
gvgehrig
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Join Date: Oct 2013
Posts: 23
Default Spongy Brakes

I'm trying to diagnose a spongy brake problem on a '41 Ford four drum setup; master cylinder with a single 1" bore is mounted lower than the wheel cylinders. Lines are bled, drums turned, new shoes, new lines, new master cylinder and tested, wheel cylinders rebuilt - no leaks anywhere. I put in a 1 1/8" bore rebuilt master cylinder, but it did not help. Anchors and shoes are properly adjusted; even tried tightening the shoes while on the lift so they had no movement. On occasion, I can get a hard pedal on the lift and for one or two hard brakes, but then it goes spongy again. I cannot consistently recreate this condition. Hydraulic brakes seem simple enough. The master cylinder pumps fluid out of the ports; the shoe springs push fluid back into the master cylinder when pedal is released. There is no check valve on the master cylinder to resist back flow, but the system is sealed, so vacuum pressure should hold the fluid in the wheel cylinders. Wheel cylinders have an internal spring to keep the seals expanded. Wilwood makes a 10# pressure valve which is supposed to keep the fluid from draining back into the master cylinder. Why is this extra valve necessary? Is it required for all four cylinders? Also, when I bleed the brakes again after getting the spongy pedal, I see no air in the lines. If fluid was draining back to the MC, shouldn't there be air in the wheel cylinder? Your thoughts would greatly be appreciated. Thanks
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