Restarting stdout for createprocessasuser to hang pipes while reading

I am using CreateProcessAsUser to start a process from a service and redirect stdout to a named pipe. But when I try to read from a named pipe it hangs on reading and never returns. I tried it in a separate task (as currently implemented below) and just read it in the main process. I also tried using the C # AnonymousPipeServerStream class (see code with comments below) but had the same file.

Any ideas what I'm missing? Thank!

            Native.SECURITY_ATTRIBUTES sa = new Native.SECURITY_ATTRIBUTES();
            processAttributes.nLength = Marshal.SizeOf(sa);
            processAttributes.lpSecurityDescriptor = IntPtr.Zero;
            processAttributes.bInheritHandle = 0;

            //AnonymousPipeServerStream pipeServer = new AnonymousPipeServerStream(PipeDirection.In, HandleInheritability.Inheritable);
            IntPtr readPipe = IntPtr.Zero;
            IntPtr subProcessPipe = IntPtr.Zero;
            if(!Native.CreatePipe(out readPipe, out subProcessPipe,ref sa, 0x0001))
            {
                throw new Exception("createpipe gave error: " + Marshal.GetLastWin32Error());
            }

            log.InfoFormat("pipe created");

            Task readPipeTask = Task.Factory.StartNew(() =>
            {
                log.InfoFormat("startingtask");
                var readBuff = new byte[BufferSize];
                uint bytesRead = 0;
                bool retval;
                while (true)
                {
                    retval = Native.ReadFile(readPipe, readBuff, BufferSize, out bytesRead, IntPtr.Zero);
                    if (!retval || bytesRead == 0)
                    {
                        log.InfoFormat("finalread {0} {1}", retval, bytesRead);
                        break;
                    }
                    log.InfoFormat("after read {0} {1}", retval, bytesRead);
                }
                log.InfoFormat("endingtask");
            });

            var processInfo = new Native.ProcessInformation();
            var startInfo = new Native.StartupInfo();
            startInfo.Size = Marshal.SizeOf(startInfo);
            //startInfo.StdOutput = pipeServer.SafePipeHandle.DangerousGetHandle();
            startInfo.StdOutput = subProcessPipe;
            log.InfoFormat("calling createprocess");
            if (!Native.CreateProcessAsUser(
                UserTokenHandle,
                null,
                command,
                IntPtr.Zero,
                IntPtr.Zero,
                false,
                0x00000020 | 0x00000010 | 0x00000400,

                IntPtr.Zero,
                null,
                ref startInfo,
                out processInfo))
            {
                throw new Exception("Create process didn't work : " + Marshal.GetLastWin32Error());
            }

            log.InfoFormat("process id = {0}", processInfo.ProcessId);
            int processId = processInfo.ProcessId;


            using (var process = Process.GetProcessById(processId))
            {
                var pid = process.Id;
                log.Info("process started : " + pid);

                process.WaitForExit(ProcessTimeoutIteration);
                log.Info("process Completed");
                readPipeTask.Wait();
                log.Info("process Completedafter wait");
            }
            ////log.Info("reading pipe canread = " + pipeServer.CanRead);
            ////log.Info("reading pipe isconnected = " + pipeServer.IsConnected);

            var standardOutput = new StringBuilder(InitialBufferSize);

            //////var readBuff = new byte[BufferSize];

            //var ac = new AsyncCallback(CallBackMethod);
            ////var bytesRead = pipeServer.Read(readBuff, 0, BufferSize);

            log.Info("before first read");
            ////while (bytesRead > 0)
            ////{
            ////    //fileStream.Write(readBuff, 0, bytesRead);
            ////    standardOutput.Append(Encoding.Unicode.GetString(readBuff));
            ////    bytesRead = pipeServer.Read(readBuff, 0, BufferSize);
            ////}