Query Corner

Q11.02. Great Western Railway vacuum

The GWR used 25 inches of mercury as the vacuum brake setting, whereas 21 inches was standard elsewhere. This situation continued into BR days and caused considerable aggravation with much pulling of vacuum release cords, especially at inter-regional junction stations where locomotives were changed, such as Bristol Temple Meads. The change to 21 inches was authorised in the early 1960s, by which time it was deemed highly unlikely that any type of train could be put together using mainly GWR stock, with the smaller GWR brake cylinders, which may have caused braking distance difficulties. When exactly did the changeover on western region locomotives occur? (MG:7487)


Engine 5659 returned from Works (Caerphilly) to Pontypool Road Shed in 1963. It had been experimentally modified to work with 21 inches of vacuum, instead of the normal 25 inches - G.W.R. standard. However, within a very short period the experiment was cancelled and 5659 was converted back, on Shed, to 25 inches operation. From this, one can but assume that this was a blanket cancellation, covering all GWR engines subject to this experiment; given that dieselisation was in full flow, and interface situations involving GWR and LMS locomotives, or whatever like situation, disappeared. (WAD: 9162)

This deals with the steam element, but what about the Western Region BR diesels delivered with a 25 inch vacuum setting, which were definitely converted in the mid 1960s? (QCEd)

The final standardisation of all locomotives and vehicles to use 21 rather than 25 inches of vacuum appears to have taken place over a weekend in about 1969 or 1970. Certainly at Crewe instructions were put in place that everything with a Western Region allocation that came on shed was to have the vacuum re-set to 21 inches. (AB)

We now have a definitive answer for the end of one of the Great Western’s many unique features. Western Region diesels were converted from 25 inches to 21 inches of vacuum between 7th January 1966 and 10th January 1966, by which time there was probably too little Great Western designed rolling stock in service to be able to compose the majority of a train and significantly reduce braking power. Advice was published in notice K2/47/65 and repeated in Traction Bulletin for Drivers No. 2 of December 1965. It is remembered that a triangle of paper was stuck on the vacuum gauge of modified locos. (JS)

last updated: 23/07/14