Serrated Goodyear actuator replacement (1948 170)

[oldtrucksrule]

I had a soft right brake that wasn't getting better with servicing so, I decided to overhaul the master cylinders. After inspection and referring to The 170 page, I realized I need to update mine IAW SNL "Master Cylinder Changes". This replaces mine with 0541138-2 and -1. My question is, where is the best place to source these? Does anyone on here have a set collecting dust? What's the collective head shed's recommendation?

Thanks, Cory

[GAHorn]

I recently suffered a similar problem in which I was embarrassed to have to admit was a simple problem…which I missed…. ….. related somewhat to the dicussion on-going about how to inspect flexible hoses.

The 172 we own developed a “soft” brake…. which I serviced with fluid because the master was …not full…. This helped for one or two flights, then the pedal needed “pumping” again before it would develop braking pressure.

No leaking fluid anywhere …. although re-servicing with fluid helped….for ONE brake application.

I lost confidence in my ability to diagnose the problem…. and handed the airplane to an A&P shop I trusted. They blamed the Lok-O-Seal and overhauled the master cylinder. They also replaced the brake linings (on that side only) stating that the linings were “worn un-evenly”.

This worked “OK”…for about two landings.

THEN…. I noticed the “flexible” brake hose on that side was …1- excessively long….2-hardened and no-longer truly “flexible”…. and…3- placing an “angular/twist” upon the caliper….which was causing an observable “space” or “air space” between the linings and the disc. (Note: Disc brakes only “relax”, no longer applying pressure against the disc…they don’t actually “retract” from the disc as “drum” type brakes do.)

My next effort at solution was to replace the hose by manufacturing a new one …the proper length which would not apply inward/outward pressure upon the caliper…and would allow the caliper to “slide” upon the locator-pins in order to remain properly positioned in relation to the disc.

FANTASTIC solution….for almost 3 or 4 landings.

It was THEN that I again jacked the airplane off the ground and …with a bright light…. gave the wheel a “spin” …. and noticed that the DISC did not run-true, …but instead had a distinctive “wobble”.
I removed the wheel and clamped up an old, condemned hollow-axle into the bench-vise and put a dial-indicator onto the disc and gave it a “spin”.

I’ll try to post a video of that …Here:


SOLUTION: Replacing the disc with New (both sides for good practice) was the SOLUTION to the problem.

Caution: When installing new discs, notice that the wheel-half has a very slight, almost un-noticeable, recessed “landing” for the disc. If that surface is not perfectly clean…and if the disc is not perfectly and COMPLETELY “seated” within that recess….the disc may not run “true” and may still create a “wobble” …which will again cause a soft pedal. (I did not have that problem of seating the disc properly against the wheel-half…I only wanted to mention it because the machined-surface “recess” may not be observed without bringing attention to that.)

While inspecting brakes…don’t forget to look for disc “run-out”, “wobble”, … “coning”. (this is when the disc is no longer FLAT across it’s diameter from -edge-to-edge- …. placing a straight-edge across the disc can reveal this.)

I hope this little story helps diagnosing a “soft brake pedal”…. when the disc “wobble” pushes the caliper-piston back into a “retraced” position during wheel-rotation, leaving an air-space between the disc and brake linings.

Addendum: I do not have a recommendation for replacement brake cylinders other than “new” from Spruce, etc. Salvage yards are likely sources but you can get what you pay-for, if you know what I mean. I’ve had good results from Texas Salvage, and ASOD (Lancaster Tx Airport)