Brake piston to bore clearance

On my recent trip to Tucson, San Diego, Oakland and return, I lost the left brake at KCCR. The o-ring extruded. Thankfully the failure occurred during the runup and not when I was landing. I note some others have had this issue.

I measured the piston to bore clearance for a used 30-9 brake on my shelf after I got home. The piston to caliper bore diametral clearance was about .010". There does not seem to be any wear evident on either the piston or the caliper bore. I could not find any dimensional information on the Parker website.

Does anyone on this forum have any information regarding the piston OD and caliper bore ID. .010" seems way too big for a nominal 1.5" bore diameter. A typical hydraulic shop rule of thumb is .001" per inch of bore diameter. Automotive brake calipers manufactured by my former employer Kelsey-Hayes used a clearance of .003 - .005" for a 1.89" (48mm) nominal diameter with a steel piston and .004 - .007 if the piston was phenolic. Larger clearance lowers the pressure at which seal extrusion occurs.

Automotive brake calipers have used square section O-rings instead of circular section O-rings for decades. Perhaps square section O-rings are more resistant to seal extrusion. I am personally aware that one of the automotive reasons to use square section O-rings is reduction of brake drag which improves fuel economy.

If any of you have any knowledge regarding the above, ,I could sure use some insight. I would like to prevent this type of failure from occurring again, if at all possible.

Larry Tompkins
N544WB -6A
W52 Battle Ground WA

I just rebuilt the Cleveland calipers on my LongEZ last weekend - not sure what model # but they have piloted piston bores and are considered mid -weight calipers for the 5:00x5 wheels. Mine are 32 years old and the clearance on one was ~.003 on one and ~.003+, almost 4 on the other after a real light session of gray scotchbrite on the piston. I proof loaded the brakes with 100# on each master cylinder overnight and the calipers remained bone dry. 010" sounds like a lot of clearance and opportunity for the O-ring to extrude.

By the way, 32 years of using exclusively DOT 5 on these brakes