Are you sure yours are f'ed?
If you've taken them off you must have altered the adjustment.
Don't forget no fluid flows through them when the rear of the
car starts to lift/under heavy braking/this is how they work.
Most people refit them without having them correctly adjusted
which simulates the rear end of the car in the air which of
course means no fluid comes through and leads Many Many
people to believe theirs are no good and need replacing.
I've NEVER come across any that can't be coaxed back into
life. But, as Oranoco correctly says above once back on and
working be very careful about setting them up/backwards
down Lea Bridge Road at 5pm on a Saturday afternoon
ain't a lot of fun..........
Hope this helps
The Dr