[HO455]

By new brakes I am assuming you mean you have installed new pads and you have replaced the three hoses? Since our trucks are 45 plus years old and that's alot of years for things to get damaged. Take the time to inspect all of the hard brake lines. By inspect I mean run your hand over every inch and look at all of it. You may find kinked, smashed, twisted, or wet areas that mean a pin hole leak. My Burban had steel lines that were twisted where the hoses hooked up and a smashed line next to one of the headers. I posted some pictures of the damaged lines on page 8 of my WMB thread if you are interested. (Link below) This inspection will cost you nothing but some time, but it may save your truck.
Also pop the top off of the master cylinder and shine a light in. You should see the nice clean fluid and be able to see the bottom of the reservoirs and they should be nice and clean. If there is a bunch of crud in there, sipon the fluid out and then carefully remove all of the debris and crud. Without getting any of it into the small holes in the bottom. I have used a hand vacuum pump to sucessfully suck the dirt out in the past. If it is bad and there is 1/8" or more of crud I personally would pull the master cylinder, tear down, clean and rebuild it if I didn't find any corrosion in the bore. If the bore was corroded I would replace the master cylinder. If the fluid is not clear it needs to be replaced by bleeding the complete system.

Good luck and welcome aboard. Every one here has been where you are. You however have this great forum for advice, whereas a lot of us learned things the hard way as it was our only option. I believe I am speaking for the group we really want you avoid the pitfalls and bad decisions so you can enjoy your truck for years to come. That is why we are willing to give up our time to help you. All we ask is to respect our input.
Marine-58 has gone out of his way to help and I thank him for his patience and contribution to the next generation of Gearheads