diff --git a/pytm/pytm.py b/pytm/pytm.py
index 211e3c6..0f31bfa 100644
--- a/pytm/pytm.py
+++ b/pytm/pytm.py
@@ -288,6 +288,12 @@ def _add_data(container, value):
             container.append(item)
 
 
+def _fill(obj, attr, source, source_attr=None):
+    source_attr = source_attr or attr
+    if attr not in obj.model_fields_set and hasattr(source, source_attr):
+        obj._safeset(attr, getattr(source, source_attr))
+
+
 @dataclass
 class _FlowDefaultsBuilder:
     """Collect and apply default relationships for data flows."""
@@ -400,37 +406,16 @@ def _merge_overrides(self, flow: Dataflow) -> None:
             pass
 
     def _apply_response_defaults(self, flow: Dataflow) -> None:
-        flow._safeset(
-            "protocol", getattr(flow.source, "protocol", getattr(flow, "protocol", ""))
-        )
-        flow._safeset(
-            "srcPort", getattr(flow.source, "port", getattr(flow, "srcPort", -1))
-        )
-        if hasattr(flow.source, "controls"):
-            flow.controls._safeset(
-                "isEncrypted", getattr(flow.source.controls, "isEncrypted", False)
-            )
+        _fill(flow, "protocol", flow.source)
+        _fill(flow, "srcPort", flow.source, "port")
+        _fill(flow.controls, "isEncrypted", flow.source.controls)
 
     def _apply_forward_defaults(self, flow: Dataflow) -> None:
-        flow._safeset(
-            "protocol", getattr(flow.sink, "protocol", getattr(flow, "protocol", ""))
-        )
-        flow._safeset(
-            "dstPort", getattr(flow.sink, "port", getattr(flow, "dstPort", -1))
-        )
-        if hasattr(flow.sink, "controls"):
-            flow.controls._safeset(
-                "isEncrypted", getattr(flow.sink.controls, "isEncrypted", False)
-            )
-        if hasattr(flow.source, "controls"):
-            flow.controls._safeset(
-                "authenticatesDestination",
-                getattr(flow.source.controls, "authenticatesDestination", False),
-            )
-            flow.controls._safeset(
-                "checksDestinationRevocation",
-                getattr(flow.source.controls, "checksDestinationRevocation", False),
-            )
+        _fill(flow, "protocol", flow.sink)
+        _fill(flow, "dstPort", flow.sink, "port")
+        _fill(flow.controls, "isEncrypted", flow.sink.controls)
+        _fill(flow.controls, "authenticatesDestination", flow.source.controls)
+        _fill(flow.controls, "checksDestinationRevocation", flow.source.controls)
 
     def _enrich_data_attributes(self, flow: Dataflow) -> None:
         for datum in list(getattr(flow, "data", [])):
diff --git a/tests/test_private_func.py b/tests/test_private_func.py
index 7f7b06e..fc74796 100644
--- a/tests/test_private_func.py
+++ b/tests/test_private_func.py
@@ -10,6 +10,7 @@
     Dataflow,
     Datastore,
     DatastoreType,
+    Element,
     Finding,
     Process,
     Server,
@@ -73,6 +74,37 @@ def test_responses(self):
         assert insert.response is None
         assert insert.isResponse is False
 
+    def test_explicit_dataflow_values_survive_check(self):
+        tm = TM("TM")
+        user = Actor("User")
+        web = Server("Web", port=8080, protocol="HTTP")
+        login = Dataflow(user, web, "Login")
+        login.protocol = "HTTPS"
+        login.dstPort = 443
+        login.controls.isEncrypted = True
+        reply = Dataflow(web, user, "Reply", isResponse=True)
+        reply.protocol = "HTTPS"
+        reply.srcPort = 443
+        reply.controls.isEncrypted = True
+        assert tm.check()
+        assert login.protocol == "HTTPS"
+        assert login.dstPort == 443
+        assert login.controls.isEncrypted is True
+        assert reply.protocol == "HTTPS"
+        assert reply.srcPort == 443
+        assert reply.controls.isEncrypted is True
+
+    def test_check_with_element_and_boundary_endpoints(self):
+        tm = TM("TM")
+        user = Actor("User")
+        element = Element("Element")
+        zone = Boundary("Zone")
+        Dataflow(user, element, "To element")
+        Dataflow(element, user, "From element", isResponse=True)
+        Dataflow(user, zone, "To boundary")
+        Dataflow(zone, user, "From boundary", isResponse=True)
+        assert tm.check()
+
     def test_defaults(self):
         tm = TM("TM")
         user_data = Data("HTTP")