how to cure hard brake pedel on vauxhaul astra 166cc 1998?

Faulty master cylinder, seized caliper or brake cylinder.

If you haven't renewed the brake fluid every 2 or 3 years there is likely to be corrosion in the system and / or perished seals.

If you've pushed back the pistons in the brake calipers without undoing the bleed nipple first (e.g. when changing brake pads) you could have flipped the master cylinder seal.

ChicksFan
March 2013