Hello, I hope the following is of assistance.
This is a common concern on the 2.0L DW 10 engine and is due to a fault with the non return valve on the vacuum pump for the servo assistance. The valve is available separately from the pump and is easy to change if you want to do this on a DIY basis. The Ford number for the valve is(NNN) NNN-NNNN
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.
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