Revs dip when braking

[clayman]

A pipe going to the servo was replaced yesterday as the old one had broke off. This morning when braking the car cut out - but i noticed that on the old pipe there was no cap on the nozzle on the pipe, so i took it off and that stopped it cutting out, but now the revs drop to about 500 under heavy braking and if you tap the accelerator and let the revs drop back down, they goto about 700 for a second.

any help will be appreciated!! :D

cheers!!