R65 dual ATE caliper issues
My wife acquired a 1981 R65 last year and that means I have another project. The bike is an old city survivor that may not have ever spent any time inside. When we picked it up the front brake lever had about half-travel before the front brakes engaged. The feel of the brakes was pretty good in that there was sufficient "feel" and they stopped the bike well. While investigating the issue I discovered that the pistons in the calipers were retracting farther than what I think they should when released. I understand the nature of the square seal and that the pistons are supposed to release a little, but this seemed excessive.
So I removed the calipers and broke them all the way down and cleaned them. There was a little sludge in there but the pistons are in good shape and the calipers are free of corrosion. I installed all new seals and put the calipers back on the bike. After bleeding the air out of the system the lever action was actually worse than before, and the pistons were again pulling away when released. After some thought, I puled the pads on one side and pumped the brakes to see if I could get the pistons to push out. I could not. They would retract back into the caliper faster than I could pump the lever.
So I took each caliper off and used compressed air to actuate each piston individually. A couple were initially a bit stiff but after being actuated once or twice they'd slide right back into the caliper with what I would describe as "proper pressure" compared to all the other brakes I've ever rebuilt. They all operated smoothly.
I put them back on the bike and bled the system again. And then again for good measure.
And they're not any better than before I took it all apart. Maybe even slightly worse.
The bike is equipped with the square reservoir cup/master cyl/throttle assy. It has dual ATE calipers, each with 1 piston on each side of the rotor. The pads look practically new and do not bind in the calipers.
I'm trying to decide if the pistons for each half of the caliper are different and I missed it. I wonder if the master cylinder is misadjusted or maybe the wrong one and doesn't provide enough volume. I'd rather not break them all the way down again, but I'm not satisfied with them, and never have been even though they work pretty well. Mostly because it's my wife riding the bike, not me. I'd probably just live with it- but IMO it's not good enough for her.
Any insight would be greatly appreciated.
Just in the last day or so there was a guy that had a similar issue being discussed on the "bmwr65.org" forum. Look in the technical section. Usually this will be related to the master cylinder and the fluid return orifice. Sometimes it get junk in it that needs to be cleaned out. You mention giving the pistons and calipers a good cleaning but double check the master cylinder just for good measure.
I suspect you have a bit of air somewhere. I have a similar R65 in the shop now and that 16mm MC moves plenty of fluid to get the pads out. My problem is the reverse in a way; the lever is way too hard but that is because of poor condition in most of the system (specifically hoses). Hopefully when all is reconditioned it will be softer, but I will let you know what I find.
What I have done in the past when I have a situation that feels like the one you have is to squeeze like crazy on the lever once I get pressure. It seems to seat the seals or something. I can remember doing this several times on the old pendulum ATE calipers.
I went through the master cylinder this afternoon and didn't find anything remarkable. The ports are clear and the cups look relatively new. I'm beginning to suspect that someone rebuilt the system not too long before we got the bike.
The thread on bmwr65.org is interesting. Ultimately it was pads in the wrong place on that bike, and he was having the same issue I am. I am positively sure that I didn't mix up the pads, but with the idea that someone else may have been in the system not too long ago, I won't count out that the previous owner may have. I guess I'll have to shuffle them around next time I have a chance and see what comes of it.
I had the chance to shuffle the pads around this morning. I think I found the best combination- they are much better. It seems like I made a major job out of something relatively simple. Interestingly, when I held a bright light behind the caliper and looked at the space between the pads and the rotors, I could see that two pads weren't worn evenly. They were fatter at the top than the bottom, and then a bit fatter at one end than the other. Ultimately then ended up on the inside of each caliper for the best lever action and feel.