Needing to pump master cylinder is sign of too small of volume - either master cylinder piston too small, or wheel cylinders adjusted too loose. When tightening the wheel cylinders, tighten them to the point where you can barely turn wheel with 2 hands, then back off until you can turn with one hand with difficulty. If you have an old master cylinder and your are pumping the pedal all the way to bleed brakes then you most likely ruined the seal on the master cylinder by having it travel over the unused rusty area. Also, if you converted to a dual master cylinder of lesser volume on each port, then you will not have enough fluid for the rear cylinders when the front cylinders bottom out. Most of the big trucks are designed for 80% braking in the rear. Most of dual cylinders deliver majority of fluid to front (assuming larger piston of disk brake). Not sure what your master is, but that needs to be kept in mind - I do know someone that plumbed it backwards on purpose for that reason and it seems to work well. My truck is a 54, so it might be quite different from yours. Also, fluid squirting up from master is not unusual and may not indicate a problem, depends on the master cylinder and its piston position when first depressed. I completely rebuilt the brakes on my 54 4100 back to stock, and everyone that has driven it is amazed by the braking power and ease. I had a lot of issues initially (wrong master cylinder, wrong wheel cylinders up front, pumped the master cylinder over rusty area and caused a leak, etc.....). Just some food for thought that helped me.
