Last year I redid the whole brake system- new master cylinder, new lines & hoses, new calipers, discs & hand brake shoes.
The one issue that's not gone away is an RPM rise when pressing the brake pedal at idle.
Having done the research and read up on the site, it's clearly the brake booster.
I also replaced the O ring between the master cylinder and the booster, it's made no difference.
I received a quote for refurbishing the brake booster which came in at approx. €280-
I eventually took the plunge and ordered an new ATE T52 brake booster for €349- from an Ebay seller with whom I've dealt with before & who's been very reliable (I have no connection with them):
http://www.ebay.de/itm/231778541563?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITInteresting to note that SLS and other online suppliers sell the same booster for +€500-
Best,
Mike