[Sirrotica]

Pin hole in a brake line is what I found with my 93 K3500 and I did the same thing you did, I replaced the master cylinder, same symptoms. No improvement and I found a brake line hole just under the master cylinder with a tiny pin hole in it.

If you haven't yet replaced all the brake lines on yours yet, your probably due. Mine started with rear lines and I just gave up and replaced them all along with both wheel cylinders.I used the Nicopp tubing (Nickle Copper alloy), bought a good quality double flaring tool (my old flaring tool was just flat out worn out) and made all the lines myself. The former owner repaired the rear lines with new prefabbed lengths of steel line just before I bought it, and they rusted out within 4 years.

The Nicopp will never corrode or rust and is easy to flare as well as bend. It's a bit pricey, cost me about $75 for enough tubing to do the whole truck, 25 ft of 3/16 inch, and 25 ft of 1/4 inch. With 100% new hard lines, flex hoses, and rear wheel cylinders, and I rebuilt both calipers. I should be good to go for as long as the truck runs, 260,000 on mine right now.