Well, somehow air is getting in. The trick now is to find out where. When you got air from the system last time, where did they come out?

 

Might be the master cylinder, or a poor seal at a fitting, or a pinhole on the line from the reservoir to the cylinder.

 

Well, somehow air is getting in. The trick now is to find out where. When you got air from the system last time, where did they come out?

Spike, you are more knowledgable on this then I am but if air was getting in, wouldn't fluid be coming out since the system is under pressure? Maybe I am comparing to a vehicle brake system too much. I know I do not need to bleed my car brakes unless I open the system or the reservoir gets to low and causes air to get in. If there is an opening (loose fitting or cracked line), then fluid will leak out.
When I bled the rear brakes on the bike, the air came from the rear caliper and not from the front. But when I pushed the pedal, neither front or rear worked. That is why I was thinking the rear master cylinder may be worn and need replaced. The brakes seem to fade gradually, after they were bled they worked great. As the week went on, regardless riding or bike sitting in the garage, they would get weaker. I could pump the rear pedal and get them back, but after about 2 weeks that would not even work. I will say I found condensation on the top of the rear reservoir cap when I removed it the first time.

 

What method are you using to bleed the brakes, gravity, pressure or vacuum?

Maybe have small air bubbles in the remaining in the system and they move to form an air pocket??? When bleeding tap on the caliper and lines to break free any air that may still be stuck in them.

Hope you get this resolved.

 

Spike, you are more knowledgable on this then I am but if air was getting in, wouldn't fluid be coming out since the system is under pressure? Maybe I am comparing to a vehicle brake system too much. I know I do not need to bleed my car brakes unless I open the system or the reservoir gets to low and causes air to get in. If there is an opening (loose fitting or cracked line), then fluid will leak out.
When I bled the rear brakes on the bike, the air came from the rear caliper and not from the front. But when I pushed the pedal, neither front or rear worked. That is why I was thinking the rear master cylinder may be worn and need replaced. The brakes seem to fade gradually, after they were bled they worked great. As the week went on, regardless riding or bike sitting in the garage, they would get weaker. I could pump the rear pedal and get them back, but after about 2 weeks that would not even work. I will say I found condensation on the top of the rear reservoir cap when I removed it the first time.

No, the MC can seal under pressure but when you let off the pedal, air can suck in around the piston seal.
I might happen at the caliper end too but I am unfamiliar with that, but I have seen it happen with drum brakes at the wheel cyl.

 

What method are you using to bleed the brakes, gravity, pressure or vacuum?

Maybe have small air bubbles in the remaining in the system and they move to form an air pocket??? When bleeding tap on the caliper and lines to break free any air that may still be stuck in them.

Hope you get this resolved.

I have a brake bleeder but I used plain old pressure method (pump pedal then open bleeder as you slowly push pedal down till it stops then close bleeder & do again till no bubbles) to ensure I go through the proportioning valve well. You only have one option for the bleeder valve on the rear and they say to use the lower bleed valve for the front two, which is what I did.

 

BUT, your rear pedal also runs the center pistons of the front calipers as well. So, if you only bled the rear caliper, there might be your problem

 

I got no clue as to the 1800, but on my GL1500 linked system you bleed the front first then the back....oh and I didnt read all the post....good luck

 

Condensation or water vapor in your master cylinder?

Since you had good pressure after the bleed it sounds like you're (hydroscopic) brake fluid is absorbing water directly from the air, either through the master cylinder or from an overly elastic or damaged brake line.

(Unless you used old brake fluid, sucked air in at the master cylinder or bleeder valve, trapped air at a banjo bolt or not fully closed the handle-bar master cylinder before flushing the rear brake/remote master cylinder, the fluid is exposed to the air somewhere along the way).

If you flush the fluid and it is clear, what color is it two weeks later? If the rear cover had the condensation, could there be a crack in the line or the plastic?

 

Update on this issue. I went for 2 months of bleeding the brakes and it was like a broken record. I would bleed, brakes worked great right after bleed. After a week of bike sitting or riding, the brakes would fade. After 2 weeks, they were gone and I am bleeding again. I finally let my mechanical senses take over and replaced the rear master cylinder. It has been 3 weeks now and they are still holding perfect. Something to keep in mind if you get tired of bleeding.

 

3 weeks now and they are still holding perfect.

That's simply the input shaft seal into master is leaking when under vacuum sucking in air vs only fluid.

If this was a car - the vacuum chamber bolted to master would be full of brake fluid for power brakes. Fairly common on older rides.

 

I posted a while back that my 03 1800R lost rear breaks. Front breaks are good. It was mentioned to me by several knowledgable folks on here to bleed them and it should fix it. I questioned that since it is a sealed system and it was not opened so how is air getting in there. Also, neither the rear or front pistons worked when the rear pedal was pushed and I would think air in the lines would still allow one of the of the two areas to work.
Well after I bled them several times I did get some air bubbles out and all seemed great. Rear brakes were solid again. But... that lasted a whole one week. Once again I have no rear brakes. So... seeking those knowledgable follks again. Do I bleed again and again or is it time to replace my rear master cylinder?

Thanks in advance,
Ray

My 08 T model did the same thing. Was on my way home from buying this scooter. I bled the brakes & it worked out fine. Must have been air in lines I reckon. I hit the rear brake deer to slow down out where I live to turn into the driveway & nothing, lol. The front brake worked fine. Good thing it wasn't an emergency huh, lol?
It's weird cause the man I had just bought it from had paper work on it where he had done work on the scoot & he had just changed clutch to a H/D clutch, the brake & clutch fluid, fork springs & fork oil, oil & filter, 2 new tires, new air filter, new plugs & Mustang seats. The only thing I found wrong was the exhaust nuts were loose at the cylinders & the rear brake. He lived out by the highway I rode it home on that was 35 miles from my place. I never used the brakes to slow it down until I got to the house. Funny he was a mechanic.