You can always vote at https://steemit.com/~witnesses if the SteemConnect link isn't working.

If the value of SBD goes down , won't it decrease post rewards and at the same time if steem goes up it will be even more difficult to power up.

If SBD goes down and STEEM goes up by the same amount then post rewards will remain the same. The system is designed for SBD to be equal to 1 USD, which is very important in my opinion. Since it hasn't been anywhere close to $1 in a while it has caused a lot of problems.

If STEEM goes up in price it will cost more to purchase STEEM to power up, but post rewards will be higher and that's generally the overall goal of this entire platform - to increase the price of STEEM.