Vintage Osmotor Running On Air - From A Steamlines 'Rheidol' Loco - Bayfields Engineering

I recently picked up a vintage Steamlines 'Rheidol' Locomotive from ANG- which needed some work.
Starting with the chassis, I have stripped all the paint and added frame spacers for extra strength.
The 'Osmotor', which had issues, was taken apart to see what was wrong.
New 'O-rings' were fitted, and the port faces lapped.
A new thicker reversing valve was made and lapped, as the old one leaked very badly.
Plenty of oil was added and so far it's running well on air.
Unfortunatley, the Osmotor unit does not run as well on steam. Still a few leaks and alot less power (any resistance appled to the wheels the the osmotor stops). Any ideas would be most welcome at this stage!