Hey Guys,
This is just an FYI, NOT a shameless plug for the S.S. De-Link KIt! Rebuilding one portion of a 37 year old braking system on ANY motorcycle just puts excess pressure on all of the other elements of that braking system, in my opinion! My "Guess" is that the rear proportioning Ball Valve is stuck, and not allowing fluid to pass to the rear caliper. That valve was such a hassle that Yamaha doesn't make it any longer for any of the First Gens. (And stopped making it almost before the end of the run of the First Gens!) It was a good idea...on paper, but the nature of brake fluid makes it a sticky mess if you don't maintain the flushes of the brake fluid. Along with the proportioning valve getting "Glued" in the closed position, are the caliper pistons on your 37 year old brak system. With the position of the rear caliper behind the engine, and trapped between the rear bags, overheating was a real issue. This is why the rear M/C looked so bad, I can't tell you how many First Gens. I have received at the shop and had to go through the entire braking system, even without adding the De-Link Kit Upgrade just to get the calipers functioning again. Almost every one! The major design element of the De-Link Kit was to take the proportioning valve out of the system every bit as much as it was to convert the bike to the right hand brake lever working both front brakes and the foot pedal working the rear brake.
There is NO rebuild kit for the proportioning valve, BUT a good disassembly and cleaning of the front left and rear caliper, as well as the proportioning valve without replacing any parts is the very minimum you are looking at here. You don't need to do the De-Link unless you absolutely can't get the proportioning valve functioning again. Also DO NOT get BrakeKleen on the Seals inside the Calipers!!! The seals will swell, and you will never get them back into the caliper bodies again!
I have ONE final tip for bleeding out the brakes on these bikes, and that is if you are not cracking EVERY Banjo Bolt and EVERY connection point starting at the Master Cylinder and proceeding to the proportioning valve, and then to the rear caliper, (or in the case of the left front caliper, the M/C, then the connection up by the steering head bearings, then to the left front caliper) you have NO idea where the blockage is and i can guarantee that there will be AIR trapped in the braking system somewhere along the line! Period!! To simplify this, crack the first banjo bolt closest to the M/C, briskly compress the foot pedal or brake lever, close the banjo bolt, do it again until ALL of the air is gone at that joint. Move to the next connection, repeat. I find this works best with a "Helper". My wife's right leg is twice the size of her left leg from stomping on the foot pedal of First Gens!! LOL!! I have several Vacuum bleeders here at the shop, and NONE of them work on these bikes! NONE! I do use a positive pressure bleeder on Autos and Trucks that works very well but I haven't been able to adapt it to the different motorcycle master cylinders. YET!
Because of my shop here in South Central Pa. I buy direct from Yamaha and am happy to share my discount with the forum members. I also offer the De-Link Kit for the members that are going to keep their bikes and want to upgrade their braking systems.
I fully understand it is very hard to justify spending hundreds of dollars on improving the braking system on a 30+ year old motorcycle that you most likely paid $800 dollars for. BUT the fact remains that you need to be able to STOP that motorcycle or you are NOT going to be able to ride it safely OR sell it safely. That's why I suggested just cleaning everything up without buying new parts and bleeding the system properly.
I haven't added it to the "Member Vendors" section of the forum yet, but I also offer a direct bolt on S.S. Line Set for the Second Gens. as well. If I can help you with new parts, or if you have other questions, or you are interested in the De-Link Kit I offer, please feel free to PM me.
Earl