Good morning, The front right brake is sticking/rubbing on our 1974 988A. We have removed the wheel, changed the liners, swapped "baloons", check retaining springs.... etc.... We tested the baloons using air to see if they properly inflate and deflate, and they seem fine. The brake still rubs. Does anyone have any experience with a problem like this? Thank you.
Are you sure only the right side is dragging? Both brakes on each axle are T'd togeather. Have you tried opening the brake bleader to see if letting pressure out lets the dragging brake release? If this works when you open the bleader for the right close it and apply brakes again and release them this time open the bleader on the left side to see if that releases the brake on the right side. If Opening the left bleader does not release the right brake I'd be looking for a problem like a pinched line to the right side. If opening the left bleader does release the right side problem may be in the slack adjuster. Most of the problems we had with those were a bad valve inside that caused a delay in the braking but if something like a piece of rubber from a failed hose got in there I guess that could cause it to not release completely. Good luck and keep us informed. Ken
Thanks for the quick reply. Yes it is only on the right side. We have tried opening the bleeder on the right wheel. It releases the pressure slowly. We have tried a pressure gauge on the line going to the right wheel and i jumps from 0 to 500psi when pressing the brakes. We also passed air through the line between the T and the wheel on the right side. If it was a slack adjuster problem, I would expect the problem on both wheels, right? Your idea of opening the left wheel bleeder could help isolate the problem (maybe a blockage in the T). Any other ideas with the information I have given you?
Have to be honest it's been a few years since I worked on a 988A, somedays wish we had them back instead of these newer machines! Yes if there was a problem in the slack adjuster it should effect both sides. Just on inlet and one outlet from it. Unless there is a blockage in a line opening either bleader should have the same result. If the slack adjuste is working right there will be some pressure in the front brakes but only enough to hold the pads ouot part way. Slack lets all the oil the brakes want to go in but control the amount that comes back out to keep pads close to the drums. Question. Was any work done to machine just before this problem started? Didn't just replace any front brake parts? Recent history sometimes will give a clue to the cause. I'll keep thinking about this as I work on other problems today!
IIRC the 988 we used to have, used a hydraulic expander tube to apply the brakes and also employed a nitrogen charged accumulator system. Hope this rings a bell for someone's memory.
I keep thinking slackadjuster too but your problem is a bit different. The slackadjusters have a sealed piston with a spring behind it and a check valve in the center of it. Apply the brakes and the oil drifts the piston to end of stroke then the check valve opens to fill the bladder. let off the brake and the springs force the oil out of the bladder untill the piston returns to where it started. If the seals leak or the piston seizes the brakes will lock on. You say you swapped the bladder, was it with a new one. The slow bleed makes me think a flap of rubber might have covered the outlet, maybe the inlet too and act like a check valve. I assume the cage and pads move free and springs are strong. Later Bob
I believe it was swapped with a used bladder... but I will have to check on that. We were also thinking about a piece of rubber blocking the outlet.... The cage and pads move freely, and the springs are strong.
Again I would like to ask when this problem was first noticed. Did you take the bleeder screw out and use a drill to clean out the mud and rust that always plugs them? If the bleeder screw is restricting the oil flow it will appear to be slow to bleed off the oil. Not saying this will cause you brake to drag but could give you a false impression of where the problem is located. One other point I just thought of. The oil that comes out the bleeder screw is from the nipple on the bladder that is at the top (12 o'clock). While the oil that applies and releases the brake comes from a nipple in the bladder 90º away at 3 o'clock or 9 o'clock. Point being that even if the bleeder nipple in the bladder did have something restricting the oil there would have to be another problem causing the oil to not vent back out the nipple that applies the brake. Won't say that it's not possible but...unlikely in my book. Worked on many 988A in the past but this is a new problem to me. Most problems I recall were connected to the tin strips on the bladder coming off allowing the brake pad backing plates to cut into the bladder or damaging the orings on the nipples when fighting the bladders on the hub. Almost forgot the broken retraction springs cutting the bladder. That's about the only wheel end problems. Slacks often would have a failure of the check valve or the seal ring on the big piston. We used to be able to replace the check valve if you knew the number to order, not shown in most parts books.
Ok... I tried a few more things... now it is getting very strange! We cleaned out the bleeder screws. We checked the T for blockage... nothing. Now we disconnected the left brake line and blocked it.... voila... the right brake works fine. Once we connect the left brake line up again... the left works and the right wheel jams again! Any ideas???!!! Thank you in advance.
Maybe the LH brake pads and drum are worn too much and are returning a lot of oil that is loading the RH brake that has less wear. Later Bob
Thanks for the quick response... I think you may be onto sometime... the pads on the left hand side are more worn than the right hand side.... Why would this load the right hand wheel though... wouldnt the excess oil go out through the slack adjuster??
As per what Bob says. Your problem is in the left wheel. IMHO a teardown of the LH side is in order. IIRC the brake system pretty much relies on the wear on pads & drums being pretty much the same on both sides in order for it to work correctly. Maybe the unbalanced wear is what's causing your problem. Also what I'd suggest is to disconnect the lines between both wheels and the slack adjuster, and blow them out with shop air just to make sure there are no odd bits of rubber or whatever from the bladders hanging around in there. If you haven't had the slack adjuster apart I'd suggest doing that as well, just to discount it from the equation. Also the lines from the slack adjuster to the master cylinder because iot appears as though something somewhere in the lines can't return the required quantity of oil from the bladders back through the m/cylinder to the reservoir. I'm trying to rack my brains regarding issues on 988A's but it was in the dim & distant past (80's). If I think of anything else I'll post it here.
Ok. We swapped out the slack adjuster.... same thing. Left wheel brakes fine.... right wheel locks up. I doubt that it is the difference in pad wear. We have always changed the pads one wheel at a time, and never have had this problem. We have contacted a couple other mechanic's... and they dont have any better ideas? Any one else have an idea!!??
Was it a new slack adjuster you installed ..? Do you by any chance have a set of pads with about the same sort of wear on them as the RH side has..?