New Girl with Old bike

[KatInTheHat]

Front brakes are sticking without engaging the brake, new caliper builds pressure but doesn’t release
To give some context, I started riding this bike not long ago and had some issues with the back drums and had to excessively use the front brakes for a short time, after which they started squeaking horribly. It still rode and braked with no issues, but after the rear brake fiasco, I didn’t want to ignore it. Upon inspecting it I saw the back pad (the side without the piston) was firm against the rotor. Me and my bf tore into it with no experience and took the caliper and swing arm apart thinking we could just figure it out (I think all we would have needed to do was adjust the spring/flathead but it’s too late). We got new pads at that point as well. We cleaned everything thoroughly with brake cleaner, greased with DOT3 and brake caliper grease put it all back together, but now the piston started sticking. Could only get it back in using a C clamp. It will move out slowly with leaver pumping but doesn’t compress again. We bought a new caliper, piston, and seal, but it’s all the same for the most part. We can push the piston down almost all the way with just our thumbs but the last bit won’t budge without the clamp. I don’t understand.
furthermore, with the new pads installed - with the piston still compressed all the way! - there is still so much resistance on the rotor and the bike is very hard to push around. Are the pads too big?? Some people say they shave them down.
Please help!

 

[madmtnmotors]

What year/model? Either way, the culprit can often be the master cylinder. There are two holes in the bottom of the master cylinder reservoir, one larger hole for engagement, and a much smaller hole to allow the fluid to flow back to the reservoir to relieve pressure when you let off the brake. This smaller hole can be easily clogged over time and can prevent the caliper from releasing. Rebuilding the master cylinder can correct this if you make sure the relief opening is clear when doing so. The circlip holding the piston is the most difficult part of the process, patience and a little intestinal fortitude will get the job done. Once that is addressed you can then focus on potential issues with pad thickness. There is an adjuster on the caliper bracket to set the spacing of the rear pad in relation to the rotor, but with the caliper assembled on the bracket with the piston fully compressed, if the gap between the pads is smaller than the thickness of the rotor then that is a different issue.

The front fender bracket can affect caliper bracket alignment as well. Some early models may have positioned the fender bracket between the caliper bracket and the lower fork, while later models, at least on my K8 CB750, the caliper bracket goes against the lower fork with the fender bracket located inboard (towards the wheel) of that, with the caliper bracket sandwiched between the fender bracket and the lower fork.

 

[KatInTheHat]

Thanks for your insight. I have a 1978 cb750K. The man I a bought it off redid everything to look like a cafe racer so the front fender was removed. It has a steel line coming off the caliper, connected to a steel braided hose (both look new and rust free) which connects directly to the master cylinder. Everything was working fine before we took it apart. I did notice while bleeding it that there was a grayish murky looking cloud(?) floating up from the bigger hole in the master cylinder. Could it be that the master cylinder got clogged up? We have ran so much fresh fluid through it, I though it would just slowly get flushed out though..

 

[madmtnmotors]

The return hole is very small, probably about the size of an idle jet. If it were clogged I don't think there's any way to "flush" it out since fluid would only flow through this very small orifice on the return stroke.

 

[MarcinMMz]

I recently had my master cylinder pump rebuilt because I felt it wasn't working smoothly, only intermittently. Disassembly isn't complicated but requires special snap ring pliers and patience. After cleaning housing, inner parts, checking the elastomers, lubricating with silicone grease, etc. and finally bleeding the brakes I can say now it works flawlessly. For sure you will see the difference