how do you undowe the nut that hold the water pump bolts that rusted up?

For me Jason C has the best answer. If the pump is being replaced anyway it doesnt really matter what damage you do to it to get it off. Usually what happens is the threads of the bolts become extremely rusted due to drop and rise in temperature around them. On old Rover Montegos and Maestro we used to just burst the pump to bits and then worry about getting the allen bolts out. Once the tension is taken off the head of the bolts they usually come out without any problem.

If there is space try to hit the head of the bolt with a hammer, the shock from the hammer blow will help you to loosen the bolt.

Failing that the next thing to try is an allen key socket, with this you can use a long strong arm to apply extra torque to the bolt. Be carefully not to open up the head of the bolt as this can be a major pain. If you still cannot get it out then if the water pump is to be replaced try to get an angle grinder on the bolt to cut its head off. Once the head is off and the pump is out you can use a stud extractor of a good vice grip to remove the stub of bolt left over.

Boil a kettle of hot water pour it over the water pump around the bolts tap the bolts with a hammer ,insert a good fitting allan key - undo never fails

If penetrating oil did not work, you can try heat. Use a propane torch with a small flame to heat the head of the bolt. However, be careful not to burn anything in the immediate area. Get the bolt hot and then try to break it free. Good Luck

I'd take a guess at what I think might work. Place a long tube over the end of allen key which you use to turn (assuming its an L shaped allen key - and assuming you have enough access/room). Then turn the allen key from the bottom of the extension (tube) you've put over the allen key. Should give you more leverage.

for penetrating oil to work as much of the rust as possible needs to be removed from the threads, with a wire brush or any other implement you can deploy, thread cleaners (available from Snap On Seeley etc) also assist in cleaning screw threads or even a thread file, depends if it can be used practically or not, a nut cracker may assist as could a small hacksaw or pad saw, a small angle grinder may also be used to cut off stubborn bolts, all these things depend on available access, I prefer to use Duck Oil, as it substituted for Plus Gas and others years ago.

WD 40

try tapping the bolt head with a hammer

undo slowly

Get a mechanic to heat the bolts up then they should come slack put copper grease on when replacing them.

A rusty bolt, you need to put oil on it, then tap it for a while until you break the rust.

Forget WD40. Use Plus-Gas and heat, and if all fails a hammer and a drift.