Dont for one minute try to compare a P6's braking with a modern car!!
When you drive a modern car everyday,it will not feel as good!
ie You cant do the same speed in a P6 as say a Saxo/Clio, and hope to stop as quick as any modern motors!!
Check your servo to manifold pipe hasnt partially collapsed,or has a leak,its dead easy to renew,and replace jubille's,also check the valve between the pipe and the servo body is working properly,these can get a little sticky if contaminated.
chrisw is right really!
You need to concentrate on fixing 1 problem at a time,otherwise you are in danger of not getting round to any of the faults you have,and I am most concerned about your wheel alignement issues!
Just look at what nearly happened to another member,with his P6's SERIOUSLY DANGEROUS :shock: bulkhead rust!
I have a feeling after seeing it and thinking more that it is related to the bushes on the top mount to bulkhead area,and as you seem to be contemplating a front spring change,then do them and also check for corrosion or flexing of the bulkhead area!!