LACK OF TANK PRESSURE?

hi there, have just returned from a trip to Scotland, going there was great, but on the way back it was it was to say the least, a nightmare. I have the talbot Express 2 litre petrol, first it started back firing, then cutting out, checked the timimg, OK. the fuel filter had fuel in it, so checked the fuel filter in the carb, no problem. stopped at a fuel station on the A74 on taking of the fuel cap i noticed there was no tank pressure, managed to pull away ok, it was only on the motorway after about 5 minutes i noticed the change in the engine sound, there was no back firing and the engine ran has normal. What could have caused the loss of tank pressure, and why is there in the first place! regards tourer35