engine revs when braking??

[Andy52]

Hi my 99 mk4 fiesta has started to high rev as you brake at junctions, in traffic ect.....

Could this be a fault with the servo , or is there something else i am missing? Have changed idle control valve, had a new coil pack and leads, and i am going to service her this week. It is the Zetec-S 1.25 engine. 

Any ideas would be greatly recieved. Thanks in advance. 

Andy.