diff --git a/synapse/replication/http/send_event.py b/synapse/replication/http/send_event.py
index 439bfbb4f..73cd3d91d 100644
--- a/synapse/replication/http/send_event.py
+++ b/synapse/replication/http/send_event.py
@@ -15,10 +15,15 @@
 
 from twisted.internet import defer
 
-from synapse.api.errors import SynapseError, MatrixCodeMessageException
+from synapse.api.errors import (
+    SynapseError, MatrixCodeMessageException, CodeMessageException,
+)
 from synapse.events import FrozenEvent
 from synapse.events.snapshot import EventContext
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
+from synapse.util.async import sleep
+from synapse.util.caches.response_cache import ResponseCache
+from synapse.util.logcontext import make_deferred_yieldable, preserve_fn
 from synapse.util.metrics import Measure
 from synapse.types import Requester
 
@@ -43,7 +48,9 @@ def send_event_to_master(client, host, port, requester, event, context,
         ratelimit (bool)
         extra_users (list(str)): Any extra users to notify about event
     """
-    uri = "http://%s:%s/_synapse/replication/send_event" % (host, port,)
+    uri = "http://%s:%s/_synapse/replication/send_event/%s" % (
+        host, port, event.event_id,
+    )
 
     payload = {
         "event": event.get_pdu_json(),
@@ -56,7 +63,20 @@ def send_event_to_master(client, host, port, requester, event, context,
     }
 
     try:
-        result = yield client.post_json_get_json(uri, payload)
+        # We keep retrying the same request for timeouts. This is so that we
+        # have a good idea that the request has either succeeded or failed on
+        # the master, and so whether we should clean up or not.
+        while True:
+            try:
+                result = yield client.put_json(uri, payload)
+                break
+            except CodeMessageException as e:
+                if e.code != 504:
+                    raise
+
+            # If we timed out we probably don't need to worry about backing
+            # off too much, but lets just wait a little anyway.
+            yield sleep(1)
     except MatrixCodeMessageException as e:
         # We convert to SynapseError as we know that it was a SynapseError
         # on the master process that we should send to the client. (And
@@ -71,7 +91,7 @@ class ReplicationSendEventRestServlet(RestServlet):
 
     The API looks like:
 
-        POST /_synapse/replication/send_event
+        POST /_synapse/replication/send_event/:event_id
 
         {
             "event": { .. serialized event .. },
@@ -83,7 +103,7 @@ class ReplicationSendEventRestServlet(RestServlet):
             "extra_users": [],
         }
     """
-    PATTERNS = [re.compile("^/_synapse/replication/send_event$")]
+    PATTERNS = [re.compile("^/_synapse/replication/send_event/(?P<event_id>[^/]+)$")]
 
     def __init__(self, hs):
         super(ReplicationSendEventRestServlet, self).__init__()
@@ -92,8 +112,20 @@ class ReplicationSendEventRestServlet(RestServlet):
         self.store = hs.get_datastore()
         self.clock = hs.get_clock()
 
+        # The responses are tiny, so we may as well cache them for a while
+        self.response_cache = ResponseCache(hs, timeout_ms=30 * 60 * 1000)
+
+    def on_PUT(self, request, event_id):
+        result = self.response_cache.get(event_id)
+        if not result:
+            result = self.response_cache.set(
+                event_id,
+                preserve_fn(self._handle_request)(request)
+            )
+        return make_deferred_yieldable(result)
+
     @defer.inlineCallbacks
-    def on_POST(self, request):
+    def _handle_request(self, request):
         with Measure(self.clock, "repl_send_event_parse"):
             content = parse_json_object_from_request(request)