Have you said which engine and year ? It would help to know as there are some differences
Ive had this problem in the past, very annoying on a hot day !
There are three possibilities :--
1. Faulty non-return valve on vacuum input to reservoir
2. Leaky reservoir, either split body or one of the pipes is cracked where it joins the body
3. Leak from vacuum pipes AFTER the reservoir.
3. is most likely, trace the vacuum pipe which runs from the vacuum tank by the pollen filter to the rubber grommit in the bulkhead where the heater hoses pass through into the cabin, follow it all the way, feeling for any flat spots or other damage where it has rubbed on something. Most likely place is where the pipe passes through the wall of the pollen filter chamber. If you find a damaged spot you could try a smear of araldite to plug the hole or cut the pipe at the leak and use a rubber joining piece from a scrapper.
If 3. doesnt fix it then you will have to did deeper. You will need to get access to the end of the pipe going to the heater, on some cars there is a T piece near the heater pipes and you can disconnect it there, if not then dont pull it out of the bulkhead grommit, you wont get it back in easily. It is better to cut the pipe somewhere accessible and use a rubber joining piece.
Suck on the pipe going towards the heater, you should be able to pull a vacuum on this.
Suck on the pipe running from servo line to vacuum tank whilst keeping a finger over the pipe running to the heater. You should be able to suck all the air out of the tank and the non-return valve should keep the vacuum until you release your finger from the outlet pipe.
If you decide that the tank is faulty, dont try to get it out as it is welded to the pollen filter assy. Aquire one from a 98> V6 which has an identical one screwed to the engine inlet trunking, just wedge it into a convenient corner.