GL1500 Rear Brake Pedal Pressure Loss

[warrengevans]

Four years ago the foot brake pedal on my 1990 GL 1500 Aspencade slowly lost all pressure. I could not find any fluid leaks so replaced the rear brake master cylinder with an OEM part from Wingstuff. Pressure returned for 6 months then slowly deteriorated to a dangerous level. Conducted complete brake fluid change but again the pressure dropped over several months. Still no sign of a fluid leak anywhere. The problem continues to occur and I have now conducted three more brake system fluid bleeds. I have very recently replaced the rear brake calliper seals but that did not solve the issue. Had the mechanics conduct another brake bleed but after four weeks sitting unridden in the garage, the rear brake foot pedal has again lost pressure. The fluid reservoir remains full with no noticeable drop in level. All work has been undertaken by qualified and experienced GOLDWING mechanics. They cannot solve the problem. This has become a very frustrating issue.
The bike is in beautiful condition and is 28 years old with only 104,620 kilometres on the clock. I have owned it since new. Has anyone out there experienced a similar problem with pedal pressure?
Does anyone know of a fix please?
Thanks,
WAZZA

[WingAdmin]

Well you've got one thing left to look at: the left front caliper. Remember that the pedal actuates both the rear caliper and that left front. If you're not losing/leaking fluid, the reservoir isn't going down but you're still getting soft brakes, it means air is getting into the system somewhere. You've bled the rear - but to properly bleed that system, you need to first bleed the front left caliper, then the rear. You've replaced the master cylinder and rebuilt the rear caliper, so time to look at that front caliper.

[Happytrails]

Would maybe suggest to double check the connections at the rear master cylinder. Had same thing happen here and thats what fix mine. It wasn't enough to let fluid escape but it was enough to let air in. Very weird. Also replaced all those sections of line to the rear caliper with a braided stainless hose.

Some else I got into routine was to put some pressure on the rear brake pedal when the bike was parked to see how solid I could get it. Just enough to put a little pressure on it and let air out. Made big difference and also on my Ural do the same thing when pedal just doesn't want to get firm. Maybe just tiny air bubbles just enough. Would repeat this until I was happy it was very solid. Then stop doing it to see if it hold. Good luck

[warrengevans]

Many thanks to WingAdmin and Happytrails. I will try your suggestions at first opportunity and let you know the outcome. Wazza.