Loosing water through my cap (DX)

Hi all,

I've been chasing a water leak which occurred after I rebuilt the coolant system last year. I replaced the water pump for a genuine VW one. I replaced all the hoses with silicone ones, the thermostat for a genuine VW one and the heater matrix.

The water seems to be escaping through the cap. I have had several caps including a new one from VW and a known good expansion tank. I am getting to my wits end as this has been going on for over a year!!!

It has blown one heater core in this time so the system is obviously pressurising although I don't know the two are linked. When the heater core blew the RAC man who rescued me did not seem to think the head gasket had gone but I'm struggling to think what else it could be. No signs of head gasket failure to mention though.

Any ideas?

Many thanks in advance. 

get the coolant sniff tested, that'll tell you if there are exhaust fumes in it. apart from that all i can think of is you are unlucky with reservoirs, try a genuine vw one the pattern ones are rubbish. if it has a level sensor have you checked that as well? they can leak thru the pins as well as past the seal