I've noticed that its common, while watching a brake application, to see the piston (hollow rod) extend an inch or two before the pushrod begins to move. What is the reason for this extra "slop"? I can see not wanting everything tight in release, but it seems that this much may be unnecessary. What, if anything am I overlooking? Is there a standard gap that should be maintained between the piston (in release) and the end of the pushrod?

Nothing standard there, Dan.

The arrangement that you are speaking of is found mostly in freight applications. The slop between the piston rod and the push rod represents the slack in the brake rigging on cars where there are not truck or body-mounted release springs. It also prevents the piston from being pulled out when the handbrake is applied, something that used to be achieved through the use of a slotted clevis on the piston rod.

Generally, if the push rod is constantly bearing on the piston rod, then the brake shoes would be dragging. The modern freight slack adjuster now governs the travel between the two. To test the slack adjuster and arrive at the optimum distance, see AAR Standard S-486 (Single-car Tests).

For cars with a body-mounted brake system without a slack adjuster, use Kentucky windage. Your mileage may vary ...

Thanks Steve! I appreciate your help.

Dan Mulvihill