Hey guys, new to the board. just purchased a 1991 seville. the issue is when i go to back the car up it is fine till you tap the breaks. The car slams to a halt and the rear wheels are locked up. when i put it into drive it releases the breaks and i can try backing up again in reverse but as soon as i hit the breaks it happens again. i am very doubtful that it is a transmission issue because the car works fine in drive and it only locks the back wheels when im going backwards (happens in neutral too). So i just jacked up the car and found some fluid coming from a line where it connects to the rear break system. could this fluid loss be the culperate? and if so any ideas on how this could possibly be DIY'd to work again? i will post a picture up as soon as i can to illustrate my point.

 

Do both rear tires lock up, or just one?
If your car has drum brakes on the rear, the problem is likely inside the drums. Possibly a failed retainer or spring that will allow the secondary shoes to expand/apply when turning backwards, and release going forward.
If yours has disc brakes on the rear, I've seen caliper bolts come loose/fall out that will cause the caliper to pivot up, and lock into the wheels, causing the same problem. It could also be a problem with the parking brake,
A fluid leak will most likely not cause this problem, but it should definitely be fixed.

I had a similar issue with an '86 Deville, where it would intermittently lock up the right rear and left front at slow speeds(split/diagonal braking system), and I finally determined the master cylinder was causing that problem.

 

Ok so i did some looking and work. I had thought that the issue was with the rear tires due to the leakage. however i discovered that the cause was due to the front end drivers side brake assembly. I found that the lower bold that holds the brake assembly in place was missing and when the car was moved backward this caused the whole assembly to shift and rub then once the brake was applied the whole thing would become pinched and cease up. after a quick trip to the hardware store for a 12mm by 1.75 course thread bolt and a new set of front end brake pads the problem was resolved.

Afterwords i decided to flush the brake line and discovered that the leak from the back left tire was due to the caliper leaking... not a big issue to me as it was very minimal and only really happened when the parking brake was pumped to engage. passed inspection because of the truth behind the saying "i know a guy", and luckily a 91 in NY state is grandfathered in in emissions. leakage is minimal at this point but once i have the funds i plan to replace both rear calipers.

 

Glad you got it fixed.