Latex: avoid that page text stretches over whole page with "twoside" option

I recently had a problem when writing a Latex document which I posted here. The solution of adding the "twoside" option worked fine for me, except that when I have a page, say the conclusion, where I have just two paragraphs, so maybe half of the page filled and I then add a "\pagebreak" for having the "reference" part on a new page, it stretches the whole content of the conclusion page s.t. it fills up the whole page. This leads to the ugly thing that the spacing between the lines and paragraphs becomes huge. I googled a bit, but didn't found any option to avoid this.