I’m not entirely sure why this morning I felt the need to grab each brake pedal and test the brakes again, but glad I did. The right brake pedal was a stiff push as expected but the left brake went easily to full compression. These brake master cylinders are at least 5 years old and they’ve been activated numerous times without hydraulic fluid since they’ve been in the plane, my bad. I took apart the master cylinder and the internal o-rings on the shaft are deteriorated. The above image doesn’t show the internal o-ring which is the most deteriorated. I ordered two new MCMCRBDS4 – MC-4 SEAL REBUILD KIT from Matco. Buna-n o-rings are used in this assembly. Mil-H-5606 aircraft hydraulic fluid or compatible fluids like Royco 782 are recommended. Better to find out about this stuff now than while taxiing for the first time.