Master cylinder for 1974 Dodge Chasis class A

It sounds like your master cylinder is leaking, you mentioned fluid leaking out the back of it. That normally would be dry rotted seals on the plunger. On the body/ housing of the cylinder there will be a casting number stamp/cast into the housing. Take that number to your local napa dealer, they should be able to cross refrence it.
Another supplier you might want to check is Fleet Pride, they're a big truck parts supplier,
Good luck, you never fail, until you stop trying