2005 Aveo Sedan has a strange brake issue and a clunk in the rear right side?

[Aveo2005]

Hi!

I am trying to fix a 2005 Aveo sedan, which has two problems I can't seem to solve.

The first is that occasionally when I start the vehicle the brakes will be stiff and almost unusable. This can be occasionally fixed if I stop the vehicle and restart it, leading me to believe it may be a problem with the anti-lock breaks. However, I am unsure and it is making me feel rather unsafe. I took it to a mechanic who said there was no problem and they couldn't reproduce it. I have a bad habit of pumping the breaks, which started after this fixed the problem the first few times. This is why I feel this may be user error. Any thoughts?

Secondly, I had a problem start within the last two weeks where if anyone sits in the back of the vehicle, a clunk can be heard. This happens at its worst when I turn left. The wheel has been taken off, and the strut seemed fine. There seemed to be a problem with the clasps holding onto the muffler and a bent clasp on the bottom, but fixing both of these has not fixed the problem. The spare tire seems to be secure, and the only thing in the trunk is a bag of emergency tools and materials that is also secure. Could it be the bushing? I am not very good with cars, and would appreciate any help in trying to fix this or diagnose it. It is a very noticable clunking noise that happens often over bumps as well, and I have flipped the seat down to see if maybe it was unsecured (although many have said it could be unlatched on the bottom of the seat. Is there any way to get under there and see?). As I said, it is only if someone sits back there, and happens occasionally with someone in the passenger seat.

Thanks!

EDIT: The clunk is coming from the right side, specifically.
EDIT#2: It is an automatic and an LS if it matters.

[petrified.rabbit]

first thing # 2..

left turn with someone in the right seat? with the body roll i would suspect bottoming out of the strut or spring. Not that you would tell, but any rear sag or anything like that? what is the weight /height difference of who is in the back seat? it could be tire scrub etc, when does the clunk happen? in the apex? before/after? while leaving after the turn?

the breaks, sounds kind of like a vacuum leak in the power assist. a slow leak, or bad check valve. making the brakes harder to push, but they will still work. next time it happens if you are in a safe area, get up to about 20 mph and slam on the brakes, it will stop, do you hear/feel the pump on the abs? after you run the engine enough (even tough it was hard when started) is the pedal easier to push, if so it reestablished vacuum which indicates the leak.

[BOOSTEDCUDA]

"The first is that occasionally when I start the vehicle the brakes will be stiff and almost unusable. This can be occasionally fixed if I stop the vehicle and restart it, leading me to believe it may be a problem with the anti-lock breaks. However, I am unsure and it is making me feel rather unsafe. I took it to a mechanic who said there was no problem and they couldn't reproduce it. I have a bad habit of pumping the breaks, which started after this fixed the problem the first few times. This is why I feel this may be user error. Any thoughts?"

I have this exact same problem, almost hit a pedestrian yesterday. Have you found a solution?