Front brake drags a little after riding

[The Voice of Reason]

Here's the problem. I changed the brake fluid on my 1977 GS 750 and everything seemed to go OK, but now when I ride it to work (about 6.5 miles) and back it into a parking spot the front brake will be dragging a little bit. It's noticeable, but not so bad that I can't pull the bike backwards. Just increased drag, not a lockup.

When I leave work and head for home, the front wheel is spinning as free as ever. By the time I'm home, it's dragging a little bit again. Needless to say, the bike is staying home until I feel like I've got this fixed, because I don't like to ride without brakes. No dirt track future for me!

The brake lever feels nice and firm when pulled, so I don't think there's air in the system, but I'll probably try to bleed it tonight to see if it gets better.

Is there anything else I'm missing?

I used DOT 3 when I flushed the old fluid; from what I've read while searching this site, I guess DOT 4 would have better high temperature performance. Is it worth it to flush again, or is a general caliper cleaning in my future?

I've done a search and can't find anything on point. That doesn't mean it isn't out there, but nothing seemed to fit these precise circumstances.

Thanks in advance for all the help!

[tkent02]

You most likely have a blocked return port in the master cylinder, when the fluid heats up it expands, can't escape from the caliper because of this blockage, and puts on the brakes a little bit. It can get more hot, cause more expansion and eventually lock the wheel without warning. Far worse than no brakes!
Easy enough to fix, take the master cylinder apart and clean it out. Should do the calipers too, so a tiny chunk from one of them can't migrate up and block the return port again.

[Nessism]

Changing the brake fluid is not good enough, you need to do a full system tear down and clean out. Replace the rubber parts inside the caliper and master cylinder if they look worn, replace the rubber brake hoses if they are still present, and use caliper grease to lube the slide pins. Do this work and you can be assured everything is in proper order. Brakes are nothing to mess around with.

[The Voice of Reason]

Thanks!

Thanks, guys! I was apparently looking at the wrong end of the system. Now that you mention it, there was a little bit of thicker crud at the bottom of the master cylinder that seemed to thin out when I replaced the fluid, so I assumed it was getting better; I guess I just stirred it up.

Voice

[Llamaguy]

I had the same thing when I bought my 85 GS550, I tried to clean it out, but it was such a tiny little hole that I could. I ended up buying a new MC (and calipers+ lines) off ebay from an early zx6r for something like $30. Fixed it completely

[mike_of_bbg]

I just had that same problem and cleaned out the return hole on Sunday. So far so good. I had to drain enough fluid so I could get the reservoir off (the hole is under a small flap of plastic), then clean the hole with a pair of pliers and a small bristle from a wire brush. Then of course bleed the whole system again. In a couple of months riding season will be over for me and I'll tear the whole thing down and rebuild the MC and caliper, and install a SS brake line.

On the additional plus side, now my brake light is actuating properly too!

[The Voice of Reason]

It's all fitting together

Interesting that the previous poster mentioned that the brake light switch worked better after cleaning the return. My front brake lever does not always trigger the brake light, at least, not unless I am squeezing pretty hard, which would seem to be consistent wit the earlier suggestions. I guess I'll order some new brake lines and plan a wrenching session to clean out the whole system.

Thanks again to everyone for the advice and remote diagnosis.