Cleveland 10-30 master cylinder - main spring length?

[kenpilot]

So I have my brake master cylinders at home to change the o-rings, and I'm really puzzled about the main spring. What I see doesn't match the instructions... My main spring is bit more than 3 1/2" long at rest, but the instructions say "Measure spring (8) for a minimum free length of 4.55 inch." (Per the diagram, part number "8" applies to 10-30, 10-30A, and 10-30D. My data plate is stamped "10-30".)

That's a huge difference! I can't believe that normal usage over 20 years would compress it that much.

Maybe a coincidence, but next sentence says "Measure spring (8A) for a minimum free length of 3.60 inch." However, the diagram shows that "8A" only applies to the 10-30C, and I very clearly don't have that! That model would have a clevis-style rod end and extra mounting brackets on the cylinder. Maybe I have a 10-30 rod with a 10-30C spring? But on all 4 master cylinders... I don't believe that either.

What am I missing here? If the spring really was 4.55" long, it would take some crazy pressure to compress it down to get the rod-end clip installed.

I did verify that with my (incorrect? shorter?) spring, there is enough pressure to counteract the tiny spring (part "11") so the resting position opens the fluid bypass valve. That's what really matters, right?

I'm using the Cleveland 199-512 rebuild kit. Aircraft Spruce says this applies to "10-30, 10-30A, 10-30C, or 10-300":
https://www.aircraftspruce.com/catalog/appages/06-01130.php

And I'm following these instructions:
https://www.parker.com/parkerimages... Brake Division/Kit Documents/199-512 Kit.pdf

 

[RWoodard]

My reply may be too late, but I just rebuilt one of my 10-30 master cylinders today. Your spring is basically the same length as my 10-30 spring. I’ve just finished the reinstallation of my master cylinder and it appears to be fully functional. I’m thinking there is some sort of typo in the manual.

On that topic, thanks for posting the manual. I did the rebuild without a manual because I couldn’t find it! I just browsed the exploded parts diagram from the manual you posted and am happy to see that snap ring 13A and “stat-o-seal” 9A do not apply to my 10-30 master cylinder. It’s always a bit disconcerting to finish a job and have parts left over!

 

[kenpilot]

Glad I could help. And thanks for confirming I'm not the only one with a strangely short spring.

It’s always a bit disconcerting to finish a job and have parts left over!

And with how expensive aviation parts are, it feel so strange to toss a perfectly good "stat-o-seal".

 

[JonJay]

Make sure you polish or replace the piston rod. If you don’t, your new seals won’t last long.

 

[kenpilot]

Make sure you polish or replace the piston rod. If you don’t, your new seals won’t last long.

Definitely. I used a sanding sponge first, to get as many visible scratches as I could:
https://www.woodcraft.com/products/norton-softouch-superfine-sanding-sponge-800-1000-grit
https://www.woodcraft.com/products/norton-softouch-superfine-sanding-sponge-1200-1500-grit

And then finished with the full course of polishing pads (1500 to 12000 grit):
https://www.woodcraft.com/products/micro-mesh-pen-sanding-kit

Perhaps overkill, but I like the mirror finish in the end. Here is before and after. I can still see a slight deformation in the piston's resting position, but it didn't seem to matter. After my rebuild, I have an absolutely leak-free unit.