[Lynn Eades]

Basically if you have a spongy pedal then the problem is air in the system.
If you have a solid pedal,(no air in system) then its an adjustment(travel) issue.
There should be a "residual line pressure valve in the master cylinder( first thing in, before the big spring)This valve is there to hold some pressure in the system to stop the pistons in the wheel cylinders from "relaxing" back, so as to keep your pedal "up" Yours may be missing yours.
Most times the problems are adjustment as per Robs input.
It may not be relevent here, but one area that many people make mistakes in, is in the fitting of the shoes, and or linings. The front shoe in each hub(all hubs) should have a long (full length lining. (this "leading" shoe is "self energising"and tends to "wrap on" during braking.) the rear shoe or "trailing" shoe, is fitted with a short lining. The reason for this is that the wear rate on the front lining is greater, and the larger area, evens out the wear over the life of the linings. Dodge acieved this with stepped wheel cylinders. This principle applies to all brake assemblies that have a single wheel cylinder, acting on two shoes, with fixed anchors. (Wagner- Lockhead, Jeep Dodge etc)
A braking "bias" is required in a system like a Jeep has, and this is accomplished by way of larger bore wheel cylinders being fitted to the front axle.
One last thing that sometimes is overlooked. Make sure the vent holes in the master cylinder cap are clear, and will allow the system to "breath" This is an "open' system which exposes and absorbs water into the brake fluid(hydroscopic?) These systems require more regular maintainence that modern "sealed systems" (the ones with diaphragms in the caps) All WW2 era vehicles run "open" systems on top of which,most are single circuit systems. In other words there is no emergency system.
If you have money to spend on your old M.V. dont take short cuts in this area.