Hi and welcome to Just Answer!
Assuming your parents actively participated in their rental activity - rental losses - up to $25,000 - may be deducted on their tax return.
Their pension is most likely taxable.
For social security benefits - here is a simple test - take other taxable income plus half of social security benefits - if the total is less than $32,000 ( for married couples filing jointly) - none of their social security benefits are taxable.
If their taxable income is negative because of rental losses - they have a NOL - net operating loss.
Generally, if you have an NOL for a tax year, you must carry back the entire amount of the NOL to the 2 tax years before the NOL year (the carry back period), and then carry forward any remaining NOL for up to 20 years after the NOL year (the carry forward period). You can, however, choose not to carry back an NOL and only carry it forward.
There are relatively complex rules of dealing with NOLs - so professional help is advisable - here is a publication if you want details and examples - www.irs.gov/pub/irs-pdf/p536.pdf
Because your parents do not have earned income (wages or self-employment income), do not have qualifying children, do not pay fo r education, etc - they are not eligible for any refundable credit - and may only recover whatever taxes were withheld form pension. They may however request not to withhold any taxes and will not receive any refund - but their pension check will be larger.
Let me know if you need any help or clarification.