Rear Brake Issue

[Elmobile]

I need some help trying to figure out why my rear brake I seizing up.
The bike is a 1990 GL1500 SE. I have rebuilt the master cylinder, replaced the steel line with SS line, rebuilt the caliper. The caliper has new OEM seals and pistons. The pins are greased and it is free to slide side way as it should.
Because I have a side car, I disconnected front and rear brake. The main port now goes to the rear brake and the pressure regulated port goes to the side car.
The system was bleed with my vacuum pump and it all appears to work fine in the shop. Went for a test ride and not long after the rear wheel was locked solid. The pedal was as hard as rock. I relief the pressure at the quick connect for the sidecar brake line and the rear brake let go immediately. I let every thing cool down somewhat and returned home without touching the rear brake. I bleed the system again and all went well for a couple of weeks. Then last weekend, it did it again! Same deal, relief the pressure and go back home.
I am sure that I cleaned the pressure relief hole in the master cylinder but I can’t understand why it build pressure in the line like that... When I bleed the line, I can see the level go down in the reservoir which tend to indicate that it is opened!
If anyone has any suggestions I would really appreciate it. I am trying not to have to remove the master again... even if it looks like it is going to happen

[SG_Jay]

Relief hole is plugged. Gotta be. Fluid is heating up and expanding, has nowhere to go but to push the pistons out.
If it's not plugged solid, there must be some crud in the line somewhere that eventually blocks it.

[Elmobile]

Yes I am afraid that I had reached the same conclusion... The system has to be sealed to build up pressure like that!
I thoroughly clean the master for what you can get at, but the pressure valve is not accessible. Probably where it comes from.
Any ideas how to get rid of this crud without having to open the master cylinder? Just dreaming I guess. Both lines are flexible now so all I would need is access to the 2 bolts, without removing the manifold is there is any known alternatives?
Sidecar is coming off for sure before I go to any other open heart surgery...

[digr]

It could be that your adjusting rod is adjusted to long, if it is the brakes will lock, don't ask me how I know.

[Elmobile]

Correct it could be but I checked it when I remove the master cylinder and it is not the case... I did not follow the Honda procedure but I recorded the length when I first took it apart and just set it up as it was. Now I have to get the master apart again and see what's happening inside... Will update when I get there. Seems that it is one thing after another but my priority never get to my GL!

[AZgl1800]

did you verify the bleed hole is clear by running the smallest guitar string wire thru it?

that bleed hole being blocked is about the only way pressure could still be building up when the system gets hot.

[Elmobile]

Indeed that is where I am now.
I am quite sure that I did clean it when I rebuild the master but the project has been on stand by for a while and some crud may have got back in the system.
The master is loose now and I am about to remove saddle and battery tray to gain access to the top of the reservoir feed hose. I have SS line now which are fairly flexible so I don't think that I am going to need to remove the muffler. Well that what's I am hoping anyway...

Any hint on getting to this relief hole?

Cheers.

[Elmobile]

Well the battery tray is out of the way but still no luck to reach the screw to be able to remove the cover holding the reservoir tube on the top of the master cylinder or the pressure relief hole which is located right under it...
So before I go ahead and remove the exhaust system including one of the manifold, would anyone have any "short cut" on how to reach it?

It would be so nice if it was possible! if not, oh well, I will have to take everything apart again. Not really looking for it