rear wheel lockup in reverse

[650savag]

Has anyone experienced their rear wheels locking up when backing down a slight incline? This is a first for me. My brakes seem to be fine and I haven't had any problems with them in a long time. I started to work a few days ago and as always, I pulled forward a few feet to allow me some room to back around another vehicle in the drive. After pulling forward about 10 feet, I stopped and put the 78 620KC in reverse and started backing out of my drive which is slightly downhill. After backing up a few feet, the back wheels would lock up and stop the truck. I could pull forward a few feet, put it back in reverse and the same thing would happen. It took about 4 tries to get the truck out into the street where I could put the truck in 1st gear and head on to work. It did not happen anymore and I can't think of what in the world would have caused that. Does anyone have any ideas?

[DanielC]

I think the rear brakes on a 620 are like the rear brakes on a 521, and need to be adjusted regularly. It may be time to take the drums off the rear, and have a look. Sometimes the lining will separate from the shoes, and it could jamb in the wheel.

If you have a little time over the weekend, you need to take a look at the brake linings.

[datsunaholic]

Yeah, as above. Having overly loose rear shoes (or a broken spring!) can make them lock up in one direction. I had teh linings fall off my 510 Wagon, and it made the brakes unpredictable and it vibrated like mad.

[datzenmike]

Yes had the 20 year old original lining fall off my Sundance (24K miles!) and it acted like a sprague clutch. Turned forward, would lock up in reverse.

[650savag]

Thanks to everyone with all the great ideas. I didn't really know where to start looking but you have given me the direction to start. I can drive the 720 until I get to the bottom of this. I will start by pulling the rear drums and checking the shoes and springs and everything with the rear brakes. I did a pretty full brake job on it several years ago and the rear shoes have around 30K on them so it was about time to start looking at replacing them. I have kept the adjusters in good working order and have kept them adjusted pretty regularly. I am glad that some of you guys had some experience with this because I was kind of baffled! Thanks again Datzen Mike, Datsunaholic, and DanielC.

[Creepy Cruiser]

I just fixed the same thing happening on my hardbody. With mine, one of the arms wasn't returning quite all the way (just a very corroded center pin). I cleaned everything up and bead blasted the corroded parts, then greased everything nicely, and now it all works perfect again.

[650savag]

Thanks Creepy. As soon as I get a chance I will check it out. Hopefully it will just be something simple as that. I will update when I get the chance to see what's going on. It may be a few days before I get a chance to pull the drums off. We have the kids and grands visiting for the holidays and I start a stretch of 12 hour nites today. Thankfully I have the old 720 as a reliable standby as my commute is 60 miles daily.

[650savag]

Well I finally got started on my brakes today. You guys were right on target. I pulled the drum off the passenger side where I had spotted some brake fluid leaking, and sure enough, both liners had separated from the shoe frames. One of them was in 2 pieces (and badly worn) and the other one was in several pieces and worn thin also. I think one or more of the pieces had damaged the wheel cylinder seals (rubber cups) so I will have to put a kit in that also. I couldn't tell a whole lot about the shoe liners but it looks like to me they both had worn thin about half the length of each. It was like half of each of the liners was still pretty thick and the other half was worn very thin. If I remember correctly, I believe I installed these about 30K miles ago. I'

m wondering if I may have had them oriented wrong. I am wondering if not keeping the brakes adjusted regularly would cause the pads to wear unevenly? I will do some reading and looking before I put the new ones on. I haven't pulled the drivers side drum yet but I plan on doing that side also. Thanks again for all the good ideas and advice.