transmission noise when breaking

[2MIZ129]

so 3 weeks ago i had a fluid change on my transmission also filter couple days after that it started acting weird one of the bolts was leaking so i took it to the shop to get it tighten up they told me that it didnt need to be filled up because he checked and it "SEEMED" full after that it started slipping and hard shifting and im guessing its just low on fluid? also the problem now is that when i hard break the transmission makes a weird whinning noise after breaking like its delayed into gear maybe from the fluid going towards the front of the car could it just be that its low and this fools are just being lazy? i even told them id pay to get it filled but they are hard heads telling me that its filled... i dont know should i just go to another shop? could it be another issue here... advice would be helpful thanks