People Hi,
Last weekend while driving so as to attend an offroading meeting I have noticed that the servo assistance on my 1994 evented CT Discovery 200Tdi was failing after the second application of brakes (stepping on the brake pedal for a second time in plain language...)
The symptom was even worse when the engine was idling.
When driving at a proper speed or with the engine running at more than 1000 to 1200 rpm there was proper servo assistance but not if the brakes were used for a third or more time repeatedly. In this case after a few seconds the servo assistance was there again but once more only for up to three application of the brakes.
I drove very carefully to the next parking space and started examining what was going on.
First thing to do was to remove the main vacuum line from the vacuum pump (on the LHS of the block while facing the engine) and spray a bit of GT85 inside (GT85 is a lubricating etc spray such as theWD40 but also has teflon in it thus providing this spray with excellent lubricating characteristics, something that WD40 is not so good at) I then checked the brake reservoir cap and found that it was nice and tight in its place. I then checked the full length of the vacuum line and its entry point to the servo. I did so by visually inspecting the hose and then also spraying a bit of GT85 and looking for any possible "sucking" of the spray at any point. While doing so I noted that the spray "cloud" was getting a bit uneven close to the area where the vacuum line has a T connector which supplies vacuum to a smaller diameter soft rubber hose that provides the heater/ventilation box inside the passenger cabin with vacuum for the recirculation valve.
So I closely inspected the soft rubber hose and found out that it was raptured at the point where the plastic T connecter was inserted into it. Apparently the years (the car has left the production line on August 1993) have taken their toll on the rubber which had cracked at that point. The car was washed thoroughly with a very strong professional pressure washer the day before and most probably the jet of water has managed to cut the dried out rubber hose at this point.
So next time you appear to be losing the servo assistance from your brakes remember to also check the soft and thinner rubber hose supplying vacuum to the heater/ventilation box on 200Tdi engined Discos.
Hope this will help somebody.
Take care Pantelis Giamarellos