I am loosing coolant on my 1999 td5 XS auto. The loss is negligable but the coolant reservoir after a run is down to minimum. If I release the cap air is expelled and the liquid returns to near normal. On one occasion the temperature guage went into red but was easily remedied by loosening the reservoir cap and expelling air. I have when topping up opened the bleed vent on the top hose. I think the pressure is not normal. Any ideas?