Does it have an adjuster on the vacuum pipe connection??? If it does then turn it down, that will tighten the spring which in turn will help hold the pressure in. Follow Sponners advice and at the same time check the spring, it my well be worn buddy.
I have noticed on many occasions that the Baileys valves are failure prone, I do not recommend these to any one.
The spring mechanism on yours must be weak, that is probably why you noticed it opening.
Take it off, push the piston in and cover the vacuum nipple with your thumb, the piston should not push down until you release your thumb. If it does release before that then you have a weak valve on your hands.
rite ive tried wot u sed and the bottom pops down then wen i take my thumb off vacuum it makes a pop sound, does this mean the valve is weak?? also would it make a difference wot spring it has in it coz i think wiv baileys u can buy different spring strengths?? if so wot strength spring would i need??
also dont think that it can be adjusted, i think the top does adjust but wen tried it made no difference
Take the spring out buddy and give it a pull to stretch it about an inch longer.
Stick it back into the valve and then slap it on the car. Take it for a quick cruise and then let us know how you got on. If it is better then before but still not good enough then you are better off getting another dump valve as the Baileys are just a waste of money as far as I am concerned. Either way buddy, let me know how you got on right here and I will advise further.