diff --git a/python/sglang/srt/mem_cache/hiradix_cache.py b/python/sglang/srt/mem_cache/hiradix_cache.py
index 1e8b838d9..158685471 100644
--- a/python/sglang/srt/mem_cache/hiradix_cache.py
+++ b/python/sglang/srt/mem_cache/hiradix_cache.py
@@ -207,18 +207,17 @@ class HiRadixCache(RadixCache):
         if not waited and self.tp_world_size > 1:
             torch.distributed.barrier(group=self.tp_group)
 
-    def _reap_completed_async_work(self):
+    def _drain_async_work(self):
         """
-        Poll outstanding async work and reap completed ones.
+        Block until all outstanding async sends are consumed, then clear.
 
-        Must be called in the scheduler thread.
+        Called at the start of each event round, so work_list holds the sends
+        accumulated since the last round. This bounds it and applies
+        backpressure when a downstream PP rank lags. Scheduler thread only.
         """
-        count = 0
-        while count < len(self.work_list) and self.work_list[count].is_completed():
-            count += 1
-        if count > 0:
-            logger.debug(f"Reap {count} completed async work")
-            self.work_list = self.work_list[count:]
+        for work in self.work_list:
+            work.wait()
+        self.work_list.clear()
 
     def _all_reduce(self, data: torch.Tensor, tp_reduce_op: torch.distributed.ReduceOp):
         """
@@ -1279,11 +1278,12 @@ class HiRadixCache(RadixCache):
         self.writing_check()
 
     def check_hicache_events(self):
+        # Reap the previous round's PP-sync sends before issuing new ones.
+        self._drain_async_work()
         self.writing_check()
         self.loading_check()
         if self.enable_storage:
             self.drain_storage_control_queues()
-        self._reap_completed_async_work()
         if self.enable_storage_metrics:
             self.storage_metrics_collector.log_storage_metrics(
                 self.cache_controller.storage_backend.get_stats()
diff --git a/python/sglang/srt/mem_cache/unified_radix_cache.py b/python/sglang/srt/mem_cache/unified_radix_cache.py
index 9514172d8..fa26bbee5 100644
--- a/python/sglang/srt/mem_cache/unified_radix_cache.py
+++ b/python/sglang/srt/mem_cache/unified_radix_cache.py
@@ -395,18 +395,17 @@ class UnifiedRadixCache(KVCacheEventMixin, BasePrefixCache):
         if not waited and self.tp_world_size > 1:
             torch.distributed.barrier(group=self.tp_group)
 
-    def _reap_completed_async_work(self):
+    def _drain_async_work(self):
         """
-        Poll outstanding async work and reap completed ones.
+        Block until all outstanding async sends are consumed, then clear.
 
-        Must be called in the scheduler thread.
+        Called at the start of each event round, so work_list holds the sends
+        accumulated since the last round. This bounds it and applies
+        backpressure when a downstream PP rank lags. Scheduler thread only.
         """
-        count = 0
-        while count < len(self.work_list) and self.work_list[count].is_completed():
-            count += 1
-        if count > 0:
-            logger.debug(f"Reap {count} completed async work")
-            self.work_list = self.work_list[count:]
+        for work in self.work_list:
+            work.wait()
+        self.work_list.clear()
 
     def _all_reduce(self, data: torch.Tensor, tp_reduce_op: torch.distributed.ReduceOp):
         """
@@ -2463,11 +2462,12 @@ class UnifiedRadixCache(KVCacheEventMixin, BasePrefixCache):
 
     def check_hicache_events(self) -> None:
         """Called per scheduler step to poll async HiCache events."""
+        # Reap the previous round's PP-sync sends before issuing new ones.
+        self._drain_async_work()
         self.writing_check()
         self.loading_check()
         if self.enable_storage:
             self.drain_storage_control_queues()
-        self._reap_completed_async_work()
         if self.enable_storage_metrics and self.storage_metrics_collector is not None:
             self.storage_metrics_collector.log_storage_metrics(
                 self.cache_controller.storage_backend.get_stats()
diff --git a/test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py b/test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py
new file mode 100644
index 000000000..6e1ba43fe
--- /dev/null
+++ b/test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py
@@ -0,0 +1,50 @@
+"""Unit test for HiRadixCache._drain_async_work PP-sync backpressure."""
+
+import unittest
+
+from sglang.srt.mem_cache.hiradix_cache import HiRadixCache
+from sglang.srt.mem_cache.unified_radix_cache import UnifiedRadixCache
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=1, suite="base-a-test-cpu")
+
+
+class _FakeWork:
+    def __init__(self):
+        self.waited = False
+
+    def wait(self):
+        self.waited = True
+
+
+class _Holder:
+    """Minimal carrier exposing only what _drain_async_work touches."""
+
+
+class TestPPSyncDrain(unittest.TestCase):
+    def _drain_fns(self):
+        return (HiRadixCache._drain_async_work, UnifiedRadixCache._drain_async_work)
+
+    def test_drain_waits_all_and_clears(self):
+        for drain in self._drain_fns():
+            holder = _Holder()
+            works = [_FakeWork(), _FakeWork(), _FakeWork()]
+            holder.work_list = list(works)
+
+            drain(holder)
+
+            self.assertTrue(all(w.waited for w in works))
+            self.assertEqual(holder.work_list, [])
+
+    def test_drain_empty_is_noop(self):
+        for drain in self._drain_fns():
+            holder = _Holder()
+            holder.work_list = []
+
+            drain(holder)
+
+            self.assertEqual(holder.work_list, [])
+
+
+if __name__ == "__main__":
+    unittest.main()