The Stovebolt Site Forums The Parking Lot Big Bolts 5700 L.C.F - Main Brake Cylinder Cap and Hydrovac Rebuild Kit

Thanks for the answers!

After further cleaning and inspection of the cap it seems it had a hex shape (could've been 5/8" size) and the embossed letters I hardly could read say "USE EIS HEAVY DUTY FLUID" and "EIS W944".
So it seems it is a different cap as on your trucks!?
Thread diameter is 1 9/16".

Seems I should try to restore it to the original hex shape...