Usally there was a valve (here in New Zealand we had the same type as on a car tire) you would bleed the air out do the biz then remove the valve from the valve stem and replace the top and seal then replace the valve and if necessary you could bring it back up to pressure with a tank on the vehicle, Anoter thought has struck me it could have been a loading coil pot as well some time in the Long runs every now and then you had to put loading coils and a check finds this
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About half way down the page.