Replaced the 3 radiator hoses on my mini and followed the process to refill and bleed (including overflow tank), but now have leak from driver's side of engine when running. Removed battery box for visibility, attached a pressure tester & pumped up to rated psi. Can't see anything leaking, or even dripping under car, but pressure gauge bleeds down in ~ 10 seconds. Any ideas?

 

Thermostat gasket is the #1 suspect. Thermostat housing or coolant temp sensor
less likely but possible.

 

If you see coolant on top of the transmission, its the thermostat gasket. Replace w new thermostat and new gasket. I recall being able to get separately but you want them both.

Leak down gauge? Having never used a leak down for a coolant issue, without a drip anywhere and the age of the R50, if its not leaking on the outside, then... inside

 

It was the coolant temperature sensor. The plastic part that the harness plugs into had lost it's seal to the brass part although still attached. Replaced the thermostat & seal also while I had the battery box & air box out.

 

I once had a leak in the hose to the coolant tank. Pressure tester wouldn't find that.

 

Check the radiator hose or any hose that runs coolat to the engine