Also be careful how much 'super' pressure you use when using the plunger. Older properties can have plumbing which is more fragile and zap, cracked pipe, flood or big leaks.
Must also warn you of a friend's experience.....she lived in mid floor of 20 story and also had drain problems, which they tried all sorts of solutions for, and eventually they got a power plunger thingie....but it was electrical I think.....and found not just a plumbing water problem but a sewage blockage,.....from some neighbour up above.....and stuff poured in.....etc....and landlord ended up in big dispute with them for putting the problem right.....he said they'd made it much worse and the new plumbing of the whole flat was now down to them....(this ended very acrimoniously with them not renewing their contract with him, even though they liked the flat)
