I converted my 91 V2500 Suburban to Hydroboost brakes. I swapped in a 91 V3500 pump with 2 return lines and a 91 V3500 Hydroboost unit and master cylinder. The truck stops fine, but the rear ABS isn't working the way it should. On hard stops, they let the rears lock up. I'm assuming the Hydroboost and larger master are creating more pressure than the ABS unit was designed to handle. This is the old style ABS with the isolation / dump valve and an ABS ECU, no sensors at the wheels. I already have the 14 Bolt FF rear with the large 1 ton brakes and a Dana 60 front with 1 ton brakes. I'm thinking I need to ask a dealer if the ABS ECU or isolation / dump valve or even the combination valve are different between the 91 V2500 Burb and a 91 V3500 1 ton truck, and if so, start swapping them one at a time. Anyone have any experience with hydroboost / ABS swaps before I hand my credit card over to the GM parts guy. Thanks, Eric M.