MG Car Repair: sending unit poping out, radiator cap, lori ann

Question
Dear Howard,
I have owned my 1979 MGB since 2002, love the car, the water pump went out several weeks ago, replaced it, I just replaced all brakes, and have noticed a hissing noise when i pump the brakes and the car wants to stop, so i bypassed the servo and am amazed at how much more power i have in the car,
Now to my problem, the sending unit popped out, cleaned the unit of ani freeze, drained the radiator, use 50/50 blend, distilled h20, hand filled from the thermostat small hole and watched the air bubbles bubble up and finally no sign of air...
Unit used approx. 1 gallon 2 pints of antifreeze, calls for 14.4...the sending unit popped out again....fans do work...the temp gauge stays in middle through all this, the sending unit was replaced in 2005,
1. the replacement the garage used is the standard sending unit that just pops in the back of the radiator...i see that there is a unit with a retainer clip to hold the unit in place.
2.how do you tell if the sending unit is back?
3. what is this happening....
4. so have said to tie the unit in place/use job weld...or just replace w/the retainer style?

please help,
Lori Ann

Answer
Hi Bob,

If it were mine, I would first pressure test the radiator cap and put a lower pressure cap on if it is tests higher then 7 PSI. The pressure can be too high even with the correct temperature.  Also get the retainer clip for the thermal sensor.

Howard