On Tuesday, July 17, 2018 at 10:38:55 AM UTC-5, Scott Williams wrote:
does anyone know the Parker/Cleveland part number for the brake Disk
used on a Grob Twin II fixed gear which used a Cleveland caliper? Glider made about 1981.
I'm aware of the Dot 3/4 compatible O-ring used in the caliper.

Thanks,
Scott

Well,
to the best of my knowledge, and the Grob 103 twin II manual, and service bulletin OSB 315-17, .
the master cylinder was a modified Magura motorcycle part, which might have been original equipment on BMWs, maybe a R100? which uses dot 3 fluid.
The original equipment caliper is a Cleveland/Parker Hannifin 30-63A, which has a Buna-N O ring( MS28775-224) for use with petroleum based aircraft hydraulic fluid,
I think it was easier and cheaper to change the caliper O ring than to change the master cylinder seals.
The "O ring swap" I mentioned was intended to ensure caliper O ring compatibility with the Brake fluid indicated in the flight manual.( dot 3)
If someone simply replaced either the caliper or just the O ring with the Cleveland called out part number, this would result in the Cleveland O ring swelling over time. That is something you don't want in a caliper.
The ID label on our Grob 103 caliper should be labeled "DOT 3" but is not (Yet).
Larry Suter, on rec aviation Jan. 6th 2015 started a thread that covered it better than I.
If I'm incorrect, please post.
Thanks,
Scott