If the revs increase it would most likely be because you also have a PCV valve fitted also. First make sure you arent running a PCV system (funny disc thing with hose out the side, plumbed off the inlet manifold). The PCV system works by creating a 'vacuum' in the crankcase using vacuum supplied by the inlet manifold. This is supposed to reduce oil foaming, oil leaks, and excessive amounts of oil getting past the rings... and also reduce emissions. This could possibly be the reason behind why theres sucking that you describe.
If you run a breather on a PCV (positive crankcase ventilation) system, it will defeat the purpose of the system in the first place, and also create a big manifold/vacuum leak, which will reduce the performance of the car and also possibly cause damage.
On the other hand if you do not have the PCV system and only a blowby hose to the air filter case, you can remove the hose and plug it at the air filter case, and fit a breather to the rocker cover. K&N make breathers and so do redline. There are some other cheap crappy types that are more for bling than anything... I have a Redline filter i got from one of my local auto parts stores. The only problem i find is when running the heater while stationary at the lights, it draws in blowby fumes which are pretty awful smelling.
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