Puzzling brake issue with CX500

[tomo]

I have posted about this on a couple of CX500 forums, but am still non the wiser, so thought I`d ask on here as I know we have some pretty clever mechanically minded members not just where GS`s are concerned either !. It has me a bit stumped, so any thoughts appreciated.

So, last year I re-built my 1978 CX500, which was my everyday transport for many years. I think she was an ex courier bike, had very high miles when I bought her in the early 90s and went around the clock a couple of years later, after which I rattled up another 80,000. Took her off the road about 10 years ago, but decided she deserved to have a re-build. Totally stripped the bike and re-built trying to keep as original as possible, With the brakes, on the CX only the front wheel has hydraulic brakes and I was particularly careful when I came to that part of the build. Callipers were stripped and very carefully cleaned, bores checked and re-assembled with new pistons, seals and dust covers, along with new pads. The master cylinder was stripped, bore cleaned and new seals installed. The only thing I could`nt do was remove the round plastic resevoir, the screws would not budge and the phillips heads just rounded off.

Once the bike was ready, I took her for the MOT test in January which she passed ok. After that, have only been out five or six times due to the crappy winter weather and road salt, but on those runs noticed there was a bit of a squeaking noise from the front wheel, which stopped if the brake lever pulled slightly. I put it down to the new Pads bedding in, or maybe a slight distortion of one of the discs.

A couple of weeks ago, the sun was out, roads dry so I took the bike out for a run. Everything was fine, engine ran great, but again that slight noise from the front. After a short run, only about 15 miles, stopped at a junction. Pulled away ok and was cruising along at 60mph for around 5 minutes when the engine note changed and the bike began to lose power. It felt like one of the pots had stopped firing, but as I was only a mile or so from home I decided to just nurse it back. Managed to get home and it stalled on the drive, then went in to get a cuppa before trying to find out which pot was out. When I came back out, started her up and the engine sounded fine. I went to push the bike into the garage and it would`nt budge. The front wheel was locked solid and also the brake lever. Feeling a bit p...ed off, left it for an hour. When I came back, the wheel was free.

Anyone have any ideas ?. On the CX forum, all I got was that the brake system needed cleaning and new pistons, seals etc, even though I had mentioned that they had been. A couple of guys thought it could be due to the smaller of the two holes in the master cylinder. Apparently quite common for the smaller one to get blocked with crud, which results in the calliper pistons not retracting. It could be, as I mentioned I could`nt get the resevoir off to fully clean below it, but I could understand it if the brakes had locked on when the lever was operated. But as I mentioned, I was cruising along for a good few minutes when the brakes seem to have locked on without me touching the lever. I can`t help wondering if that noise from the front is connected, but can`t see how.

Anyway, think I will replace the master cylinder with a new one anyway to at least be sure that is taken out of the equation. But it is bugging me, any thoughts guys ?

[Grimly]

That is a classic symptom of the tiny relief / return hole being blocked.

[tomo]

Thanks Grimly, that was the opinion of most of the guys on the CX forum too, so think it must be the problem. Only thing I can`t understand though is why the brakes would apply themselves like that while I was actually riding. I did of course use the front brake to stop at a junction, so could understand it if they had locked on there, but I had been riding a good few minutes before it happened. Wonder whether if the pads were catching, making that noise, whether the fluid in the calliper could heat up. Probably talking out of my arse there, but wonder if there was a bit of crud in the mc that blocked the relief hole I guess if brake fluid can expand when heated that would cause the problem ?. Anyway, I will change the master cylinder for a new one and see how it goes.

[Gorminrider]

Im sure they told you this before at cx forum but Why buy new? Unplug the reief hole. Easy. Get a bristle from a wire brush or a guitar string or a straightened spring and poke it clear. Stiff steel in a smaller dia of the hole is all you need

[rphillips]

Go up top to search & put in... Brakes applying their self...some discussion there. There's another thread somewhere with things I've done to try to fix mine, but I couldn't find it. I hope mine is fixed as it hasn't caused a problem lately, but I haven't rode it a lot of miles since I last tinkered with it....Hard to conclude if fixed because from the first time it was "very random" when it was going to mess up.

[tomo]

Thanks guys, appreciate the advice. Must admit I wanted to keep the original master cylinder, so could be the problem area as although I cleaned out the bore, I could`nt get the plastic resevoir off as the screw heads just chewed up, so I could`nt clean out beneath it or access the two holes to check them. I am still puzzled though as to why the brakes applied themselves. I will check out the other posts, forgot about the search feature.

[Grimly]

It doesn't take much friction to start the pads heating and the fluid to start expanding, especially if you've recently applied the brakes and the pads are still in close contact with the disc. The rubber seal around the piston is supposed to retract the piston, but old seals don't do that very well any more.
So, fluid expands up the pipe and normally it would go through the relief hole and you'd never know any of that was happening. Until one day it stops working properly and the brakes bind on.

[salty_monk]

Yeah it's a vicious circle once that hole is plugged.

[Gorminrider]

Those nylon reservoirs are a bear alright...they powder and crack in the sun. I'd wonder if a master cylinder from a GL500 would fit-they are alloy and more common. There's some chinesium cheapies on ebay that actually work fine on a GL...what I did was get one and just strip it for the piston seals and the brake light switch I needed for the original master but the chinesium ones do work ok.

[sacruickshank]

Did you use correct DOT brake fluid? For example, DOT3 boils/expands at lower temps than DOT4, so using DOT3 in a system designed for DOT4 can cause over heating and brake locking. A short term fix is to crack the bleeder to relieve the pressure, but the real solution is to replace the brake fluid.

If you swap with a brake from a GL500, make sure to get one from the standard rather than Interstate. The dual disk interstate probably has a higher volume master cyl. It will work (mine does), but the front brake doesn't have much feel between off and on.