So my 1977 V12 XJC has what appears to be a brake assistance problem. Once in a while, especially at the start of a trip, the brakes don't appear to react well, and a second 'shove' of the pedal is needed. After that it's fine for a while but may recur. I've checked all the rubber hoses to the servo and the vacuum reservoir are in good shape and replaced any that looked in the least bit tired. I have vacuum pump with guage if anyone knows what else I should be checking or what other factor might be causing the poor response.

 

Master cylinder needs replacing?
Brake calipers starting to stick?

 

+1 on the master cylinder failing as a strong possibility

And/or air in the system

Cheers
DD

 

Agreed and check the one way valve in the vacuum line to the booster. I've had good luck with the "replacement" masters on amazon, and terrible luck refurbing the proportioning valve. Have removed said valve and am eliminating it as per ser 3 cars as we speak ...after I finish repairing the paint damage on the inner fender and frame rail (a real bicth).