Hot tap air lock - help needed!

Hi guys. Its probably best to explain as much as possible - please bear with me.

I recently put a new basin and wc into a first floor bathroom. The basin has a monobloc tap (two taps before). The hot water to the basin now doesn’t flow. I figured an air lock and as the cold feed to the basin is mains pressure I disconnected the pipes from the tap and used the mains pressure to force water into the hot tap feed pipe (each of the supply pipes now has a check valve fitted so did this below those). This works but only for a very short time. I’ve tried this several times and even had someone in the loft to make sure the water was being forced up through the vent pipe and into the water tank. The only way at the moment that I can get water from the hot tap at the basin is to turn it on, then turn on the bath hot tap then after about five seconds turn off the bath hot tap and hey presto the hot runs in the basin tap. But shortly after turning off the basin hot tap the air lock reappears again (if indeed it is an air lock).

I assume that for some reason the bath hot tap is helping to draw the water from the cylinder which consequently helps to feed the basin tap. I assume that the hot pipe work reaches the bath before it is teed off to the basin (and probably kitchen down below). The customer swears that she never had the problem before and I’m now stumped as for an answer to get rid of the problem. The bathroom is on the same floor as the hot water cylinder (next door room). Any suggestions as to what the cause might be and therefore possible solutions to the problem would be greatly appreciated.
Many thanks in advance.

 

Hi thanks for that Oliver. But the problem seems to be there even when the tap is not connected to the pipework

 

Hi Captain. Yes, the tap instructions specified that check valves need to be fitted on both hot and cold pipe work.
Thinking about it, once I eliminated the air lock I reconnected the tap and this is when the air lock re-established itself. It might be an idea to clear the airlock again and leave the pipework for a while. If the water still flows later on then there must be a problem with the tap and / or check valves. However, if the air lock is back then it has to be something else. Problem is, what might this something else be?

 

Also, forgot to say that the hot water on the bath doesn't run terribly fast - better in the kitchen. So the pressure in thbathroom probably isn't very good.