Re: Brake lights stay on

Posted by HH56 On 2017/2/25 11:00:06

The Hydrovac that Stude used on the 57- 58 Packards has a hydraulic seal arrangement and leather around the power piston configuration much like the Treadlevac units which Detroit built Packards used. As we have found with the Treadlevac units, years of neglect, a faulty seal, or failure to change fluid can cause problems.

Upon release of the brakes, the power piston is returned to its home position by a large spring. Any fluid which may have leaked past the seal into the vacuum side combined with old and possibly hard or contaminated lube can cause the leather seal around the power piston to gum up and stick to a point where the spring is unable to return the piston to the full release position.

In a different arrangement but much the same principle as the Treadlevac, failure of the piston to return fully home leaves in this case, a check ball rather than a valve closed and a thru port inside the slave or secondary master piston assembly is blocked. With fluid trapped inside that secondary master cylinder and piston being unable to return to the main master and reservoir, some pressure can still be applied to the wheel cylinders. Results in effect would be much like the hill holder failing to release its intentional mechanical block.

Even if the power piston side is OK and releasing properly, if the old fluid has managed to congeal or even crystalize as has been found so often in the Treadlevacs after long periods of sitting, the inner valving of the slave master piston can be prevented from working with the same result being trapped fluid.