'85 250R rear brake issue

[250rAL]

Issue: I've only got about 50% of braking power at the rear. It will lock up on gravel but not on pavement.
Back story: This is a machine that I only ride once or twice a year. A couple years ago, it started loosing brake fluid in between yearly rides. I've never figured out where it goes. There are no visible leaks but the reservoir will just be bone dry. The point is that the system has been empty in the recent past. This year it wasn't empty but I topped it off and bled it. I've now bled it several times and even reverse filled it from the bleed screw but I still can't get full power. The pedal is firm. I checked the caliper for function and the pistons extend just fine. That just leaves the master. It's never had anything done to it, never been off. I've been religious with flushes so it hasn't been neglected.
Question: Can a bad master cause this symptom?

[El Camexican]

The first thing that comes to mind is that your brake line might be deteriorating internally. Try squeezing it tightly with one hand while you work the lever with the other. If you feel anything more than a slight tightening of the line you should change it.

I’m not familiar with your model, but usually the rear brake cylinder on a bike has an adjustable rod in it. If the rod is not adjusted to allow the piston to return far enough down for the pressurized fluid to pass back into the reservoir your brake can lock up when the fluid gets hot. As well, if it’s out of adjustment you may not be getting enough fluid entering into the cylinder from the reservoir to completely fill it and that would affect the amount of pressure that can be achieved.

Check these two things first and keep your eyes open for a possible leak.

One other thing. When bleeding make sure that your bleed screw is higher than any loops or bends in the brake line and that it’s not below any part of the caliper or your never get all the air out of the system.

[shortline10]

I would just rebuild both the master cylinder and the caliper , new fresh brake pads and be done with it .
Purchase shindy kits if available .

If you have deep pockets esr sells a 450r conversion kit that gives you extremely good stopping , I just installed one a couple weeks ago .

[250rAL]

The first thing that comes to mind is that your brake line might be deteriorating internally. Try squeezing it tightly with one hand while you work the lever with the other. If you feel anything more than a slight tightening of the line you should change it.

I’m not familiar with your model, but usually the rear brake cylinder on a bike has an adjustable rod in it. If the rod is not adjusted to allow the piston to return far enough down for the pressurized fluid to pass back into the reservoir your brake can lock up when the fluid gets hot. As well, if it’s out of adjustment you may not be getting enough fluid entering into the cylinder from the reservoir to completely fill it and that would affect the amount of pressure that can be achieved.

Check these two things first and keep your eyes open for a possible leak.

One other thing. When bleeding make sure that your bleed screw is higher than any loops or bends in the brake line and that it’s not below any part of the caliper or your never get all the air out of the system.

-Hose feels fine; no expansion that I can feel.
-I don't believe the push rod is adjustable. It appears (in exploded drawing) to just have a ball end that fits against the piston.
-For the last bleed that I tried, I hung the rear if the caliper up so the bleed screw was highest and tapped with a hammer. No air came out.

[El Camexican]

-Hose feels fine; no expansion that I can feel.
-I don't believe the push rod is adjustable. It appears (in exploded drawing) to just have a ball end that fits against the piston.
-For the last bleed that I tried, I hung the rear if the caliper up so the bleed screw was highest and tapped with a hammer. No air came out.

If there's no adjustment and your hose is good, skip straight to Shortline's suggestion and rebuild what you have.

[250rAL]

The plan for now is to replace the hose with new (ordered from Streamline) and if that doesn't do it, rebuild the master. Thanks for the input!

[250rAL]

An update for anyone interested: The new brake hose didn't do it. I ordered a rebuild kit for the master but I screwed up. I didn't try to get the master apart first. I just dug into it today and found the boot on the bottom full of rust. It's going to be a challenge to get the snap ring out, in the condition it's in. Soaking in PB Blaster now...

[350for350]

I hope it all works out for you.

[250rAL]

Another update: I got the master rebuilt, which was an adventure in itself. It made no difference at all. I also removed the slides in the caliper and cleaned and greased them The rear one was really stiff and I thought that might be it. No dice. The only thing I haven't touched is the caliper and I really resist rebuilding it because I'm convinced that it's fine. I probably have no choice but to do it.

[shortline10]

New brake pads ?
If the rear caliper is leaking fluid that’s when it needs rebuilt . The master cylinder is what makes the hydraulics and stopping power but if the pads aren’t centered on the disk because the guides are stiff or stuck it will not stop very well .

Also when you rebuilt the master cylinder did you make sure the bleeder hole was free and clear ?

[250rAL]

New brake pads ?
If the rear caliper is leaking fluid that’s when it needs rebuilt . The master cylinder is what makes the hydraulics and stopping power but if the pads aren’t centered on the disk because the guides are stiff or stuck it will not stop very well .

Also when you rebuilt the master cylinder did you make sure the bleeder hole was free and clear ?

The pads are about 75% and they are even end to end and from pad to pad. Inside of the master was as clean as new. The caliper isn't leaking and the pistons both retract easily and will extend past where they sit now if you lift it away from the rotor. That's why I really don't want to rebuild it. I did the front caliper when it started dragging but this one seems fine. I don't know where else to look.

Now... bleeder hole in the master? The only holes I saw are the ones where the reservoir hose and the brake line attach.

[shortline10]

When you had the piston out of the master cylinder their is a tiny hole next to the larger one that releases the hydraulics , if it’s clogged it won’t build up the correct pressure .
If it was me I would replace the pads as well , I have see near new old pads with crappy stopping .

[250rAL]

Is that tiny hole at the end of the cylinder where the brake line attaches or on the side where the reservoir comes in? Does it go to the outside? This is all new info to me. Thanks for helping.

[shortline10]

The tiny hold is located approximately half way down the cylinder

[250rAL]

OK, I took it all back apart. I looked down the bore of the cylinder and couldn't for the life of me see another hole. On a hunch, I took off the fitting where the reservoir hose attaches and sure enough, there was another hole there. I tried to stick a wire through and it wouldn't go. I eventually shined a flashlight into the hole and looked down the bore, then I could see it. On the outside, it is one hole, on the inside it looks like about three extremely tiny holes. That's why the wire wouldn't go through. Anyway, the hole is clear.
I put everything back together and I'm going to bleed it over several days to get all the air out. When I went to drain it to take it back apart this morning, more air came out. It had been air free yesterday.