Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

Unless you already have replaced the seals, you can get these o-rings from Grainger.  I use two for each side - after cleaning the fittings (which is a lot of fun).

O-Ring: 010, 1/4 in Nominal Inside Dia., 3/8 in Nominal Outside Dia., 70 Shore A, Red, Inch, 100 PK
Item 1REC5Mfr. Model U38883.006.0025

https://www.grainger.com/product/O-Ring-010-1REC5