Why is this happenings I'm?

"I always have to fill the water pressure up the pressure goes right down on the gauge" - it shouldn't be doing that. Pressure is going SOMEWHERE

"and when I fill it up it leaks out the over flow outside" - It shoudn't be doing that, either.

The pipe that leads to the outside, relieves overpressure in the system, via a pressure relief valve (PRV).

It looks like you have a problem with yours. It shouldn't be letting anything out unless you get too much pressure in the system.

You really ought to have a Gas Safe certified engineer have a look, but these are some results I found that may help:

http://www.diynot.com/diy/threads/baxi-maxflow-prv-not-a-problem-just-a-query.121801/

It may alsp be wrth having an engineer check the expansion vessel is properly pressurised:

You can test the expansion vessel but can be quite tricky to get to. This must be done with no pressure on the water side of the vessel. This can be done by releasing pressure at a radiator bleed or drain valve. The PRV could be opened to achieve this but that is NOT a good idea as it may cause the valve to be more susceptible to leaking in the future.

Turn of the boiler, then locate the schrader valve (it looks like a car tyre valve) and should be on the top or bottom of the expansion vessel, unscrew the dust cap and press the valve inside. If water comes out the valve, the vessel needs replaced (Or a new external one plumbed in). Its dry then you need to check the pressure inside the vessel, you can do this with a tyre pressure gauge, you will have to check your manual for the pressure but it should be between 0.8 and 1.0 bar. If its low it will give you the same symtoms as a faulty PRV as when the water heats there will be no room for expansion and the pressure will rise until it opens the PRV. Anyway if the pressure is low you can top it up with air from a pump and test again in about a week to see if the pressure remains.

This may help as well. It may also give you the correct presure to use:

https://www.manualslib.com/manual/947244/Baxi-Maxflow-Combi-Wm.html

This may be useful:

http://community.screwfix.com/threads/faulty-expansion-vessel-in-vokera-boiler.123815/

One final thing - your question was really difficult to understand without meaningful punctuation.

XXX
December 2016