There are many posts all around the SOHC forum that talk about front disc brakes that stick or do not retract when brake lever is released. The evidence is obvious. The bike is hard to push around or you sense the drag when coming to a stop. Another symptom is the front disc rotor gets very hot and discolored. This article attempts to consolidate all the tips, tricks and techniques that took me a considerable amount of time searching throughout the forum and putting to practical use.
* In most cases purchasing an aftermarket front disc brake rebuild kit and replacement pads will be the best way to fix the front brake sticking problem.
* If you want to trouble shoot it as is then try these various techniques:
1. prop bike up on main stand. Have your girl friend or wife sit on back part of the seat. If she's heavy enough, this should raise the front tire off the ground. (I'd avoid any comments to her at this point). Alternatively, use a small jack and a piece of 1x board to gently jack up the bike at the exhaust pipes just enough to raise the front wheel off the ground. If it doesn't free spin or is stuck then you have the sticking brake problem.
2. first try to adjust the swing arm left to right tolerance. Use a 10mm wrench to loose nut and turn slot screw in or out until front tire moves freely. If that doesn't do it then your in for a bigger job.
<== 10mm nut for disc caliper brake swing arm on right......14mm nut for removing caliper on left
3. loosen (but don't remove yet) 10mm nut on brake line going into the front disc brake caliper. Catch any dripping fluid and wipe off painted surfaces.
4. remove both 14mm bolts that clamp the caliper left side and right side sections to the disc brake swing arm.
5. finish removing 10mm brake line fitting nut and drop inside and outside halves of the caliper.
6. from the left side of the caliper, empty out brake fluid from inside piston housing through the brake line hole.
7. remove brake pad (puck). If it's stuck there may be your problem.
8. wrap the left side of the caliper in a towel and on your work bench with the piston facing down, blow a short burst of compressed air into the brake line hole. Be carefull here. There are horror stories about blowing the piston out of the housing at high speeds break things along the way. Save your face at all costs....wear your helmet with shield down
The gentle application of compressed air should force the piston out and a bunch of messy brake fluid as well. If it doesn't come out with air you can try the grease gun method which is more safe but leaves you with a yuk of a mess to clean out of the caliper housing and passages (see Stuck Caliper Piston in Tips and Tricks section). Another method I've used in extreme cases where the brake has heated up to very high temperatures due to sticking, is heating the outside of the caliper housing with a propare torch. Get it hot but not hot enough to warp the housing or melt the silicon-rubber seal inside. Use welders gloves and once good and hot, it should blow out with compressed air. I've seen very badly corroded calipers where I had to use both heat and a rubber mallet to tap the piston out.
9. once the piston has popped out, check the exterior sides for pits.
<=== clean out this groove to make sure the ring seal seats flush inside.
11. Check the groove for deposits and corrosion. Likely there will be something there.....just enough gunk to raise the seal high enough outta the groove to prevent the piston from retracting and causing the sticky brake symptom.
12. you can use a dremel tool with a brass rotory brush to clean out the insides of the groove.
13. clean the surface of the inside edge of the caliper where the brake pad/puck would sit. Use 800 grit w/d sandpaper.
<== clean this surface to prevent puck / brake pad binding.
14. some of us reuse the old pads if they are still reasonably good. Regardless, if you're going to use your old ones or new, turn the puck around backwards to make sure the pad material fits inside the groove you just polished. I have found that sometimes the pad has distorted to the point where it catches on the inner surface of the caliper hence causing the sticking problem. If it doesn't fit without binding, use a file and remove material from the pad until you can slip it in and turn it 360 degrees without binding.
<== insert puck / brake pad backwards to check for clearance.
15. Once you've got your surfaces clean, you're ready for reassembly. Coat your new rubber seal with brake fluid and insert inside the ring groove of the caliper. May sure it seats correctly and is not twisted (that would be hard to do but you never know). Apply a bit more brake fluid around the ring seal and insert your new or cleaned up piston back inside the caliper. The solid end goes in first. It should slide in fairly easily by applying pressure with both thumbs pushing it inside.
16. Install new or reused brake pad on top of piston metal end in first. If you have squeaking front brakes, this is the time to put a dab of brake anti squeak between the piston and the puck (this works for me). Make sure you use just a bit and don't get any on the surface you just cleaned.
17. Inspect the inside caliper half to make sure the pad wear is within tolerance (note the red line). Now is the time to clean up that half prior to re-assembly.
18. Reassemble in reverse order, reconnect your brake line and fill your master cylinder 1/2 way with brake fluid.
19 Bleeding front brakes is another whole topic with dozens of ways to do it best. I've tried a lot of them including using one of those Mighty Vacuum bleeders. I find that I spill more brake fluid if I'm trying to recover the old fluid so I just bleed letting it run out the bleed screw down the caliper and into a plastic pan. If you wipe down the fluid off the caliper face with mineral spirits, the paint isn't effected. My method: Leave M/C cover on loosely and pump slowly at the brake handle. Hold pressure on the handle and open the bleed screw. Don't let go of the handle before re-tightening bleed screw. Repeat over and over until no more air comes out just fluid. Check your fluid level in the M/C between each cycle. When only fluid comes out and you have enough brake resistance at the handle, tighten down bleed screen and top off M/C fluid and tighten cover.
20. Now the test....with the front wheel off the ground, you should have free spin on your wheel. Apply the brake and it should stop. Release the brake and you should have free spin again. If you need to reseat the pads.......take a thin blade putty knife and tap that in between your rotor and brake pad on the outside. You may need to readjust the swing arm to find that perfect spot that keeps the pads from contacting the rotor.
Note: I have inquired via the SOHC forum about front brake caliper piston differences between the years. Apparently, the piston part number for CB750F '76
CB750A '76, CB750A '77 - '78, CB750K '77 - '78 is different from the earlier years. Keep that in mind when ordering. However, in what seems like an exception to the rule, my very stock and unmodified 1978 CB750K has the same sized pistons that are used in all the early year SOHC CB750Ks.
