Burlington, Ontario-based Marsh Brothers Aviation has received STC approval from Transport Canada for its Beech brake master cylinder upgrade kit, while work continues towards obtaining FAA approval in the near future.

Sandy Thomson, Marsh Brothers' founder and pilot, listens to aircraft owners and mechanics to learn about the common problems they face in order to develop better products that are tailored to solve these issues. In 2019 when Sandy met Bob Ripley, the senior technical advisor at the American Bonanza Society, on a tour through eleven Eastern American Beech repair centres, he asked him: “If there was one part that we could make for Beech aircraft in our world of bearings and seals, what would your suggestion be?”

Ripley replied: “If you could make a master cylinder piston rod seal that would allow maintenance shops to rebuild their own brake master cylinders, it would save the owners a lot of money. Currently, we have to buy a brand-new cylinder from the maker. O-rings and circlips are readily available but unfortunately, rod seals are not.”

When Thomson returned to the Marsh Brothers' shop, he presented this idea to the team and set things in motion.

“The challenge in this case was that it had to fit the Beech cylinder and have virtually the same outside dimensions as the OEM seal. The lip would need have a different profile in order to prevent leaks. The seal would also have to be of better quality and be certified,” says Thomson.

Nicholas Choo-Son, director of business development at Marsh Brothers Aviation adds: “Once we heard from Bob that there was a need for certified brake master cylinder seal repair kits, it was pretty much a no-brainer as this was in line with our business model, to provide a solution to a known issue affecting in-service aircraft that is better than the original design, and it makes a stronger business case to undertake the investment associated with an STC project. The challenge we faced and overcame had everything to do with the size of the seal in the brake master cylinder. Previously, the smallest lip seal we produced was roughly two inches in diameter. The Beech brake master seal is less than ¾ inch diameter. As part of the development process, we used digital tools such as finite element analysis to get ourselves comfortable that a lip seal this small would work in this application. Then, during development testing, we subjected the seal to test conditions that were more extreme than its typical operating conditions, ;all of this just to be absolutely certain that our smallest seal produced to-date would perform.

“When it comes to the piston rod seal in the Beech brake master cylinder, nobody pays much attention to it until it leaks. It won't cause much more than carpet damage even when it does leak, unless you ignore it. A single Thorseal in these new kits replaces two elements of the original design, which offers the same benefits over a traditional O-ring; elimination of typical failure modes while providing improved cold weather sealing performance.”

The brake master cylinder upgrade kit also marks an important milestone for Marsh Brothers Aviation, as it continues to develop innovative products solving known maintenance issues, while expanding its offerings to the Beechcraft King Air family.

The Marsh Brothers brake master cylinder upgrade kit comes with all the parts needed to rebuild a master cylinder including static gland seal, piston head seal and back-up ring, new retaining clips, nut and cotter pin. The kit even includes specialised tooling developed to aid installation of the Thorseal. Two kits are now available for sale; one for the Beechcraft Bonanza and Baron OEM 96-380034 series brake master cylinder assembly and one for the King Air and Beech 1900 OEM 90-380001 series brake master cylinder assembly, under the Transport Canada STC.