diff --git a/library/grow/pump.py b/library/grow/pump.py
index 6c73f19..200f991 100644
--- a/library/grow/pump.py
+++ b/library/grow/pump.py
@@ -45,8 +45,12 @@ class Pump(object):
         """Set pump speed (PWM duty cycle)."""
         if speed > 1.0 or speed < 0:
             raise ValueError("Speed must be between 0 and 1")
-        if not global_lock.acquire(blocking=False):
+
+        if speed == 0:
+            global_lock.release()
+        elif not global_lock.acquire(blocking=False):
             return False
+
         self._pwm.ChangeDutyCycle(int(PUMP_MAX_DUTY * speed))
         self._speed = speed
         return True
@@ -57,11 +61,10 @@ class Pump(object):
 
     def stop(self):
         """Stop the pump."""
-        self.set_speed(0)
         if self._timeout is not None:
             self._timeout.cancel()
             self._timeout = None
-        global_lock.release()
+        self.set_speed(0)
 
     def dose(self, speed, timeout=0.1, blocking=True, force=False):
         """Pulse the pump for timeout seconds.
diff --git a/library/tests/test_lock.py b/library/tests/test_lock.py
index 8901bd6..aedccc9 100644
--- a/library/tests/test_lock.py
+++ b/library/tests/test_lock.py
@@ -1,6 +1,16 @@
 import time
 
 
+def test_pumps_actually_stop(GPIO, smbus):
+    from grow.pump import Pump, global_lock
+
+    ch1 = Pump(channel=1)
+
+    ch1.dose(speed=0.5, timeout=0.05, blocking=False)
+    time.sleep(0.1)
+    assert ch1.get_speed() == 0
+
+
 def test_pumps_are_mutually_exclusive(GPIO, smbus):
     from grow.pump import Pump, global_lock