diff --git a/src/json/builder.cr b/src/json/builder.cr
index 0d8d2017d1af..d848d954bc4e 100644
--- a/src/json/builder.cr
+++ b/src/json/builder.cr
@@ -53,6 +53,7 @@ class JSON::Builder
     when DocumentEndState
       # okay
     end
+    @io.flush
   end
 
   def document
diff --git a/src/mime/multipart/builder.cr b/src/mime/multipart/builder.cr
index 4ee45c4c6d45..202406880103 100644
--- a/src/mime/multipart/builder.cr
+++ b/src/mime/multipart/builder.cr
@@ -184,6 +184,7 @@ module MIME::Multipart
       end
 
       @state = :FINISHED
+      @io.flush
     end
 
     private def fail(msg)
diff --git a/src/yaml/builder.cr b/src/yaml/builder.cr
index cc925fc5a107..acc011719d58 100644
--- a/src/yaml/builder.cr
+++ b/src/yaml/builder.cr
@@ -72,6 +72,7 @@ class YAML::Builder
   # Ends a YAML stream.
   def end_stream
     emit stream_end
+    @io.flush
   end
 
   # Starts a YAML stream, invokes the block, and ends it.