SKODAKA05

i had issue with my power steering pump. bought a new pump, Vika brand and got it fitted at a garage. i'm facing following problems after installation of new pump, 1- when i steer, steering is sometimes hard and sometimes super soft. 2- no noise on starting car but after about 1 min, some sound starts and lot of air bubbles come. also fluid in reservoir seems boiling ( kind of back pressure) 3- whining noise gets louder when driving at slow speed and steering gets harder. spoke to mechanic who installed the pump. he claims pump is damaged. on enquiring with him, he told he did not flush out fluid and just topped up, as he felt its still good to use. please share related information to help me understand the cause and solve out problems. Thank you everybody for your help.