The most likely cause of the problem is errors in the eng file. Have you checked the OR log?
With some old MSTS loco then you might not ever be able to release the brakes in OR unless you drive using simple controls. The reason is that some MSTS eng files have a lot of brake "garbage" in them - usually a mixture of air brake and vacuum brake parameters which totally confuses OR - because it makes no sense in terms of the real life loco.
Also, not all eng files have updated instructions, many are copy and paste. If you are using a GWR loco with an OR eng file such as this one:
CTN Hall Class Loco
You should follow the instructions below.
( There is also a tutorial activity for using GWR vacuum brakes on the CTN website -
CTN GWR brakes demo activity )
This is the procedure to release the brakes:
(1) Move the brake handle to the OFF position using the ; key
(2) Turn on the (large) ejector using CTRL+J
(3) When the brakes are released Turn off the (large) ejector using ALT+J - it wastes a lot of steam if you forget this.
(4) Drive off - the GWR vacuum pump will maintain the vacuum when you are moving.
You can download OR eng files for most GWR designs here -
GWRLocos.zip (382.58K)
Number of downloads: 254
You might want to compare the brake sections of those files with other GWR locos - or simply just use them for your own GWR locos.