Why would you want to change this?
My car developed a loud rattle that sounded like it was coming from the head. It would start off if you revved the engine, and returned to idle. There would be a tapping sound for about 3-6 seconds and then it would go quiet again.
I took my car to see Mechanical_Repairs so he could hear this noise and he instantly said Vacuum pump - but nothing to worry about. He checked it was still creating a vacuum by removing the rubber bung from the hoses nearby and you would feel it was sucking (circled yellow in post 3).
The noise became too annoying for me and seemed to coincide with my car starting up then stopping.
After reading pages 39-41 of the service guide charlie had referred to in another thread, I found out that leaks in the vacuum system can be responsible for frequent stalling/cutting out, high idle speeds, erratic idling, poor idling, knocking, engine starting issues, overheating, CO levels being too low, lack of power at high speed and misfires!
So if leaks can cause that then I suppose a broken vacuum pump could also cause some of those issues as well. As the part is about 180 pounds from peugeot it would not be the first thing I would recommend replacing to fix the above issues, but Carl told me that it is the same Vac pump on all models of 306 so finding a second hand one shouldn't be too difficult.
The Vacuum pump is actually used in the braking system.
So thats the blabbing out the way - on to the FAQ!
Vac Pump 4565.52 (cheers Cosmic Spanner)
(optional) large O ring seal 455605 - 53p
(optional) small ring seal 455607 - £2.51
6mm Alan Key
Cross Head screwdriver
Im not sure if new ones come with the o rings on them. I replaced the ones that were on the second hand pump I bought.
First leave your car to stand for a good few hours. As this is in the head, you must allow oil to return to the sump otherwise it will be very messy!
Use a small screwdriver or fingernails to carefully remove the old seals from the pump. The little one is a bit fiddly and snapped while I was removing it. The replacement seals fit on easily.
justinaccurate has attached the following image: