disc brake staying on.?

First a little info before my request for help.
OK, it's an oldish (1975) Honda motorcycle. Please don't go, it could so easily be a car problem too.!

This machine has been standing for many years, and I'm trying to get it roadworthy. The front hydraulic disc brake system was in a hell of a mess with the bottom of the master reservoir somewhat crystallized. I have thoroughly cleaned this up, but have not removed the master piston assembly, as special tools are needed. It works very smoothly however, with no leaks.
I have managed to get the caliper (single pot floating type) freed up nicely, and running sweetly, but when the brake is applied, it stays applied.
I have tried to push the slave piston back, but even with a 'G' clamp it didn't budge. the brake lever at this time became rock solid.
If I loosen the banjo union at the master cylinder, the slave piston returns without effort.
Any ideas, as to why the fluid will not return?

There are two holes in the bottom of the master reservoir in line with the piston.
The one furthest away from the union is about 2.5 mm diameter and the fluid seems to be pumped from here.
The other, is a recessed hole smaller than a pin prick. I am assuming that this is where the return fluid should come from.?
I can't believe that it's a simple blockage, as the pressure I have put on it should have cleared it (?)
I would appreciate any help with this.
Thanks in advance.
Flashbang.

flashbang said:

This machine has been standing for many years

Click to expand...

That is your main problem. At least renew the brake fluid, and really you should strip the lot down and replace all seals too. However, I would try cleaning the slave piston first without removing it.(yes, a bit bodgy I know but has worked for me several times, especially on the later Honda inboard ventilated disc systems,(eg, VT500, VFR400, VT250) which are notorious for seizing up and overheating). As follows :-

Remove caliper.
Operate brake lever, in order to push piston out, as far as you dare without popping it out altogether!
Clean piston with non abrasive material. I always found the backside of a strip of wet and dry to be ideal for this. Nice and strong, but not harsh. Use a touch of WD40 by all means, but make sure you wipe it all off afterwards as it will just attract more crud onto the piston. A good blast of compressed air will do no harm, assuming you have access to a compressor.
Push piston back into caliper.
Repeat. Twice!
Reassemble and refit.

I can't see your problem being at the lever end, but specify your model and that may help more! good luck.

Thanks keyplayer.
I have already stripped down the slave cylinder 2 or 3 times now. I have also replaced the seal. The piston is in pretty good shape, not perfect, but not too bad.
The slave cylinder is definitely not the problem. The piston slides in and out perfectly.
I have, of course, relaced the brake fluid. And i have cleaned out everything, including all pipe work, with my compressor.... And wd40.
I'm sure it's got to be the lever end. If I pump the brake lever the slave piston moves out freely. It's just that when I try and push it back, it won't.
If I loosen the banjo union, or undo the bleed valve I can push the piston back in with my thumbs. Leave it connected to the master cylinder, and I can't even force it back with a g-clamp.
The model is a CB250 G5. (yeah I know, a dog of a bike 30 years ago!!) but I love her.!
Thanks again.
Flashbang

Mmm, have you taken the lever assembly apart and cleaned it,ie the pivot pin/screw - maybe this isn't allowing the lever to fully return? From what you say, the master cylinder appears to be functioning ok, and I have known properly working bike brakes to have a stubbornly returning slave piston, after all it doesn't have to go far to free the disc up.

One other suggestion I can make is make sure the pads are not too tight in the caliper. Several times I've had to file excess paint off the edges of the pads when new to stop them jamming up against the disc, and coat them with copper grease*. failing that, it must be master cylinder assembly - what special tool is required, a pair of circlip pliers should do to disassemble master cylinder?

And this maybe a bit too obvious, but you haven't overfilled fluid reservoir?

*Top and bottom edges of the pads that is, not the friction surfaces!!!

Thanks for getting back to me Keyplayer.
No, the reservoir, is not overfilled.
The lever is also free to travel.
The piston that the lever pushes on, seems to be returning to the stop, but I can't confirm this. Very good point, and one worth taking a closer look at..... thanks.
Good thought regarding the pads, (or in this case pad, as it only moves one) but again it is free moving. It was a bit tight, but as you say, this was just paint, and needed a little smoothing.
The special tool needed to strip down the master cylinder, is in fact only circlip pliers. But, the circlip is in such a deep recess, that it would have to be fitted with very long points. I have 3 pairs of circlip pliers and none of them will come close.
It looks like I may have to manufacture some extra long points out of something.
Do you know if the fluid returning from the caliper when brake is released, is supposed to come back through the same hole in the reservoir as it's pumped from. Or is the other smaller hole used for the returning fluid?

The fluid doesn't really return to the reservoir until you push the slave piston back into the caliper. The 2 holes are to supply the primary and secondary cups of the piston assembly respectively with fluid, which you are pressurising when you apply lever. The cups prevent the reservoir from being pressurised.

Assuming your wheel and disc are correctly aligned, I think you're gonna have to fashion a couple of pins for your pliers and service the master cylinder. Its the only thing left!!! Maybe there is a bit of dirt trapped in one of the cups. You could try shifting it by flushing the master cylinder with fresh fluid, and vigorously pumping piston whilst pipe is disconnected, but otherwise you'll have to strip it down. Best of luck!

OK Thanks.
When /if I manage to get it apart, I'll let you know if I find anything.
I like the theory about a bit of dirt trapped.
The reservoir had all sorts of bits in it, and there's a good chance that some of this has found its way into the piston cavity. If the piston can't fully return, then I can see how that would cause the problem.
Many thanks again for your help.
It helps to bounce around ideas.
Much appreciated.
Flashbang.