diff --git a/collects/mzlib/port.ss b/collects/mzlib/port.ss
index 6a723c3..e2304f4 100644
--- a/collects/mzlib/port.ss
+++ b/collects/mzlib/port.ss
@@ -15,7 +15,7 @@
       (make-output-port 
        name
        always-evt
-       (lambda (s start end non-block?) (- end start))
+       (lambda (s start end non-block? breakable?) (- end start))
        void
        (lambda (special non-block?) #t)
        (lambda (s start end) (wrap-evt
@@ -84,12 +84,11 @@
        (semaphore-peek-evt lock-semaphore)
        (lambda (x) 0)))
     (define (read-it s)
-      (parameterize ([break-enabled #f])
-	(call-with-semaphore
-	 lock-semaphore
-	 (lambda ()
-	   (do-read-it s))
-	 try-again)))
+      (call-with-semaphore
+       lock-semaphore
+       (lambda ()
+	 (do-read-it s))
+       try-again))
     (define (do-read-it s)
       (if (char-ready? peeked-r)
 	  (read-bytes-avail!* s peeked-r)
@@ -111,12 +110,11 @@
 		      (when (null? special-peeked)
 			(set! special-peeked-tail #f))))])))
     (define (peek-it s skip)
-      (parameterize ([break-enabled #f])
-	(call-with-semaphore
-	 lock-semaphore
-	 (lambda ()
-	   (do-peek-it s skip))
-	 try-again)))
+      (call-with-semaphore
+       lock-semaphore
+       (lambda ()
+	 (do-peek-it s skip))
+       try-again))
     (define (do-peek-it s skip)
       (let ([v (peek-bytes-avail!* s skip peeked-r)])
 	(if (zero? v)
diff --git a/collects/mzlib/thread.ss b/collects/mzlib/thread.ss
index b722e4d..d839ead 100644
--- a/collects/mzlib/thread.ss
+++ b/collects/mzlib/thread.ss
@@ -73,13 +73,13 @@
   (define dynamic-enable-break
     (polymorphic
      (lambda (thunk)
-       (parameterize ([break-enabled #t])
+       (parameterize-break #t
 	 (thunk)))))
   
   (define dynamic-disable-break
     (polymorphic
      (lambda (thunk)
-       (parameterize ([break-enabled #f])
+       (parameterize-break #f
 	 (thunk)))))
   
   (define make-single-threader
@@ -113,7 +113,7 @@
 		  (let ([c (make-custodian)])
 		    (parameterize ([current-custodian c])
 		      ;; disable breaks during session set-up...
-		      (parameterize ([break-enabled #f])
+		      (parameterize-break #f
 			;; ... but enable breaks while blocked on an accept:
 			(let-values ([(r w) ((if can-break?
 						 tcp-accept/enable-break