The air rail on the left bank has a hole in it and the check valve has completely
disappeared.

In an effort to generally clean up the engine I understand I can remove these
and restore at a later date, but what I am interested in is WHY there is so much
rust.

 

The air injection system is exposed to exhaust gasses ...and water is a normal byproduct of internal combustion always found in the exhaust, so....there you have it! Just like your mufflers and pipes, the air injection systems can and do rust out.

Cheers
DD

 

Understood

I mistakenly thought it was fresh air
Thanks for the information

Coxy

 

You are not mistaken. The pump DOES supply fresh air to the air injection pipes....but the pipes go into the exhaust ports of the cylinder heads....thus exposure to exhaust gasses. The one-way check valve separates the fresh air side from the exhaust port side. 'Tis the side constantly exposed to exhaust that rusts out. Unless, of course, the check valve fails...which does happen from time to time.

Cheers
DD