I have a 1955 Mack B61 and as far as I know everything is origional. I rebuilt the brakes a few years ago with new rear drums cast by dura brake. I have the rear brake pots with the big diaphrams and they lock on low air pressure. I don't know what these are called. I don't believe they are spring brakes. Does anybody have a diagram of the system with a discription of how it works. I pulled up to a stop sign the other day fortunatually going very slow pulling my tractor and when I applied the brakes to stop I just kept rolling. I could hear the air and everything seemed ok and I believe they are adjusted ok, I'll check that tomorrow. I put a air pressure guage on the rear brake line and couldn't get more than 60 PSI. I also have a knob on the dash, that turns, that I think adjusts the air to the front brakes. I don't think my front brakes have ever worked. I'm a slow and careful driver.
So, if anyone out there can help with information thanks.
oh!, I've had this truck for about 25 years and its been the most reliable vehicle I've ever owned.