So I'm getting a random problem with breaking on my 1977 Daimler Double 6 XJC. The brakes appear to work fine but then randomly need a very heavy shove to come to a stop. After that they work fine again for a while then its shove/panic! Anyone had this issue and knows the cause?

 

It may be an issue with the brake booster, or a vacuum leak.

 

At idle when you press hard on the brake pedal, does the engine start stumbling?

 

No. If anything the revs rise VERY slightly.

 

You may have a partially/intermittently collapsed vacuum hose feeding the brake booster.
The hose may externally look OK but internally is coming apart.
I had a similar problem in a S3 XJ6 many years ago. A new length of hose is as cheap as chips

 

Thanks Bill, I will check. I assume you mean the servo or do you mean the reservoir under the wing?

 

Bill
Both hoses