On my 1988 Jaguar xjs my brake petal feels very hard, to stop I really have to give it a lot of pressure for a quick stop. The petal doesn't sink but it doesn't really move much pressure till at the end of the petal. I replaced all pads, rotors, and fluid. is it the booster or can it be cause by another series of problems.

 

Rusty/stuck caliper pistons, rusty/stuck master cylinder piston, collapsed brake hoses. This can be from age and use....or from long periods of non-use.

My experience is that a failed booster doesn't give a hard pedal in the sense of very little pedal travel. You'll have to press the pedal harder, of course, but it'll have normal movement/travel

Also, some brake pads just require a lot of effort. I had a set once where I thought I was gonna bend the pedal before getting any "bite" from the brakes. I switched to a softer composition pad. And....some pads have a specific bedding-in procedure which, if not done, will result in low stopping power. The typical bedding-in is several very hard stops from about 40 mph with maybe 30 seconds in between. Don't lock the wheels and don't come to a full stop. But, some parts have a different procedure.

Cheers
DD