There is on the thermostat housing but leaving the car running up to temp and leaving the rad cap off has always been fine for me, keep topping it up slowly. Im sure the workshop manual showed a method of holding open the rad cap to expel the air etc but the way I have done it seems to work fine.
Now Shagging sheep on the cheap
You wont find one that's for keeps
Spend the money, find a honey
'Shit' think I need to sell my Sunny!
But woman cant replace a sheep
Oh I wish that for the peace!
But I really cant find my honey
The only one I would give my money
Is one with a great big fleece
And then I would be at peace...