non expert question re bleed rads - top up water

[mandub]

hi,

sorry if this is the wrong place for me or for this question - just ignore if so.

I've checked the FAQ re bleeding rads and 'How to restore the pressure in a sealed heating systems' but as a DIY phobe wondered if anyone can help me further.

Have an Elec boiler and currently running OK but 2 of 5 rads are cool at the top. This time last year I bled a couple of them and the boiler shut down.
I had an engineer out (landlord pays for it) and it was fixed, he said the water pressure (I think) had dropped too low.

I've had a look on here at various threads but want to check before I try bleeding the rads again in case the boiler stops and I have to get an engineer out again.

If after I bleed the rads the boilers stops again how do I add water or water pressure back.
Is it the black tap in this pic I've taken - http://i51.tinypic.com/i1flhk.jpg
It's dark - so hard to see the pic well - sorry.
The connection it's on is also here - wider pic - http://i54.tinypic.com/2n21yl5.jpg
The pressure shows currently as - http://i56.tinypic.com/wbewe9.jpg
I'm not sure what the black and red dials indicate so if anyone can say what each is?

Question is really, if I bleed rads and it stops the boiler do I just need to turn the black tap from my first pic and if so until what point, am I checking the red or black dial from pic 3 and at what point should I turn the tap off

 

[45yearsagasman]

Simply turn on the black tap and watch the black needle rise to near 1bar and the turn off the tap.The red pointer is just a reference point and can be moved with a screwdriver to the position equal to the black needle and you can then see if the pressure drops over the course of time.Do all this when the system is off and is cool.

 

[mandub]

45yearsagasman - thanks very much for taking the time to reply.
Appreciate it