Working on Brakes (especially Rear Drums) - TJ

Hi Bob:

Drum brakes have a "self-energizing" action while braking in forward mode. The brake shoes actually "dig into" the drum harder when braking in forward motion. There is no "self-energizing action" in rear motion braking on drums. So when braking when backing down a hill for example you have almost NO rear braking action. Applying the parking brake will help but you want to be very careful doing that as that can easily lock up the rears which might cause complete loss of vehicle control.

If you are in 4-hi or 4-lo on a part-time 4-wheel drive system like the TJ or YJ, the front and rear axels are mechanically locked together and the front brakes will actually stop the rear wheels through the drive train. It's very important for good vehicle control on off-road hills to be in 4- wheel drive, preferably low range. That way the engine does most of the braking and brakes all 4 wheels through the drive train and you can just apply gentle pressure to the brake pedal to control the descent WITHOUT locking up the wheels and loosing control.

Tom

Hey I tried it with my 88 XJ with it down on the icy ground and with the brake pedal fully depressed the rear wheels would still spin. It would do the same jacked up in the air also = keep spinning. This was with a remanned master, new rear wheel cylinders, shoes & pads with all hydraulics bled well!

My parent's 88 XJ will now lock up the rears. I had to do their rear brakes so they would lock up. The rear wheel cylinders were "locked"/rusted up and the wheels would keep spinning either in the air or on the emissions dyno.

I would want to check my 88 XJ's proportioning valve. Waiting to get my 'round-2-it'! lol

later, dave AKA vwdoc1

How did you manage to mess up their rear brakes so they would lock up?

You need to address theirs before you worry about yours that work right....

There 'IS' a reason that proportioning valve is in there!

Crap, in an emergency braking situation, locking the rears means instant

360's with total loss of control unless you really know what you are doing.

Been there, done that a 'LOT' of times on purpose using the E-brake when I used to ice race a front wheel drive Mini.

I could drive straight down a snow/ice road and just for kicks or show in a competition hit the E-brake and do 2, 360's, let off the E-brake and continue on in the exact same track I started from.

Mike

dave AKA vwdoc1 wrote:

Are you referring to the little "star" adjustment thingy or whatever? How do I adjust this? I seem to remember seeing the guy at the brake shop stick a screwdriver through the backing plate somewhere, but I don't want to just randomly stick a screwdriver through one of the backing plate holes without knowing what I'm aiming for ;-)

Thanks

I do appreciate the comments. However, I do understand the utility of the low range for slowing down the movement of the axles. In the case I described, though, the issue would have occured regardless of whether I was in high or low range, as it occurred when I was going forward, then had to stop and put the clutch in. If I had been in

4low, I still would have rolled backward in the same way when I stopped my forward motion and put the clutch in. It's not like I would've tried to hurry and put it in reverse to use the engine braking; I expect, when I'm at a near standstill trying to go forward, that if I push in the clutch and the brake at the same time to stop, I'm not going to randomly roll back ten feet if I don't slam the thing into reverse and pull the clutch out as some sort of preventative measure.

Your statement was not bad advice by any means, just not applicable to the situation I was in. (This trail was not difficult, either; I'd been in 2wd for awhile so I could move faster; I don't like running in

3rd and 4th gear while in low range...my 4.56 diff gearing makes it difficult to keep up on an easy faster paced trail if I stay in low range sometimes.) Sure, I probably should have shifted into 4-low before trying the rocks (I could tell I'd probably not make it in 2-high), but I was in the mood to play a little and give it a shot in 2wd for kicks. I was surprised by the resulting brake crapping out when I had to give up and wanted to shift into 4-low, though.

/Bob

If your brakes are working, they will self adjust when you go in reverse and hit the brakes.

The state of the emergency brake handle tells you the adjustment usually. If it is higher than normal, the rears could need adjusting or replacing.

Otherwise, you can jack up the wheel and remove the rubber grommet and turn the star wheel until the drum starts to grab. You then back it off a couple clicks, this takes a second screwdriver poked inside to release the top bar.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Bob wrote:

Bob:

I think my statement was applicable to your situation. You said you were in

2-WD, so you could move faster. You really should have been in 4-HI, that way you could still have moved just as fast, as long as you were on dirt, gravel, mud, etc., (anything but hi traction pavement.)

If you had been in 4-HI, your front and rear axels would still be mechanically locked together through the drive train, even when you put the clutch in. Applying the foot brake would have transferred braking power from the fronts to the rears, thus preventing that uncontrollable slide to the rear with the front brakes (only) locked, even if you had the clutch in.

Tom

Open diffs mess up that theory fast unfortunately.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

mabar wrote:

I didn't get that from your post, that you were suggesting I had been in 4wd instead of 2wd, but still high gear. Yes, you do have a point there. Even if my ARBs aren't turned on, it would still help keep the rear drive train from turning if the front is locked-up by the brakes and I had been in 4wd. An interesting thought.

Regardless, just FYI, I had this same thing happen a couple weekends ago when I was backing up off a rock pile to change my line (and was in 4-low), and tried to stop. That kind-of messes up the theory a little bit. I think the fact my diffs were open at the time and that I was on much rougher rocks and mud contributed to that idea not working in that situation.

Either way, it is an interesting thought. I just like running 2wd when I'm in high range for some reason. Maybe your idea here would be a reasonable argument for using 4wd even in high range...

CAVEAT If they are working properly they will adjust in Reverse. The self adjusting mechanism often times does not function properly. Other than that, what Mike said.

You mean a round tuit? :)

Peter Parker did pass the time by typing:

That just goes to show that I don't have one......yet! lol Have a safe 4th everyone. later, dave AKA vwdoc1