1984 GPZ 750: The Ferrous Mistress,

[pablohoney]

The fluid is pretty clear, but one thing that is getting to me, is that when I open the bleed valve, it all loosens up, which would seem to point to it being pressure holding it. Plus it was only stiff when the bike had been sitting. But it seems to be fine now, doesn't seem to be sticking, I will keep an eye on it, but if I don't have any problems with it, I am not gonna worry about it.

 

[MotorbikeBruno]

Yeah, that pressure is usually from the piston (pads) pushing against the rotor. Just be careful sir. When I was newer to the older bikes, I had a set lock up on me. Luckily I was coming to a stop...just happened a tad more abruptly than I thought. Was able to open the bleeders, and use the back brake to get home. Keep the rubber side down!

 

[pablohoney]

I have found that it only tightens up when it is hot in the garage. Has been fine for days, but spontaneously today when it is hot, it tightens up. Possibly the fluid expands and cant go back into the reservoir and creates pressure?

 

[MotorbikeBruno]

Yeah, usually due to water or crap that gets in the lines. Bleed the living hell out of it and try again. Still think it's dirty caliper though. And remember that it will tighten on you as you ride, because it gets hot while braking...

 

[pablohoney]

Well, I think you may be right, I took a ride and it tightened up a bit, and got the rotor nice and hot, I will just pull the rear brake and clean the caliper real good. Thanks for the help.

On a side note, some moron tightened that frigging fork cap down to about 200ft lbs! Was a nightmare to get off. Some people just shouldn't be around motorcycles.

"Moron 1" How tight should I make this? manual says 16.5ft lbs."
Moron 2 "Screw the manual, get that 4ft breaker bar and tighten it as much as you possible can!"
Moron 1 "She-it Yeah!"

 

[MotorbikeBruno]

Just looking out for another DTT brother that's all 8)

And yeah, My favorites are the guys who tighten things until they go POP and then sell that bike to you...ugh. Good luck on cleaning your brakes.

 

[pablohoney]

yeah, that is always fun. Haha.

I think you were onto something with the dirty caliper, or perhaps piston. can I sand this off, or do I just need a new piston? Btw, what is up with that inner part of the piston?

Think some brake cleaner ought to take care of the caliper, but i think I am gonna need an exorcist for the piston.

 

[MotorbikeBruno]

Your piston might be toast. Looks super pitted. If it's not in the travel space the piston uses, it's fine. If not, well time for a new piston. The condition of the "inside" of the piston means nothing, it's really just a dirt collector and for weight savings I suppose (rather than having a large chunk of metal that wasn't hollow).

Use a wire brush and get as much corrosion off as possible on the piston. The caliper looks good inside, just clean thoroughly and you'll be fine there. Take the rubber o-ring gasket out and note what side was "up" as some of them had directional o-rings, not sure if yours does. Best thing I found to work to get all the grime out of the o-ring groove was a wire brush on a dremel, it fits in there perfectly and cleans it out in a matter of seconds. Used to use a tooth brush or brass brush and brake cleaner...took forever and never was really that clean

So for now, clean it all up, put it back together and see what happens. Be on the look out for a new piston, and realistically a "brake rebuild" kit with a new o-ring as brakes are rarely something you want to Ef with using old parts hoping they'll work at speed...

 

[pablohoney]

I got the piston pretty well cleaned up, think it will be ok. Want to get new seals though, a bit messed up. The piston moves pretty well without the seals, but with, it is mighty sticky. Ideally how well should the piston move in there?

 

[MotorbikeBruno]

Well, it needs to have the seal in there (o-ring) and it should go in (with the bleeder open) kinda smooth, so long as there is (fluid/brake rebuild grease? can't remember the name of it). If all was clean and you are trying to push it back in with just brake fluid, it will be a bit tougher. Push it until it bottoms out, then close up the bleeder, and push the lever (a billion times while bleeding the brakes to get that last bit of air out), it should come out nicely, open bleeder, and push back in, this time it should go in easily and shoot out a bunch of fluid from the bleeder, if not, replacement time for sure but also verify you cleaned it REALLY well.

That's how I do it, but if anyone else has another way, chime in.

 

[pablohoney]

The bike is just about done!

I found an extra piston I had in the shed which is in much better condition, and then I threw some new seals in and Voila! The brake works great!

Also brought the bike to a shop and got the carbs tuned, apparently I did a horrid job, anyways now it runs great!

 

[pablohoney]

Got the mistress all fixed up, running well and sold! Twas a good educational build.

 

[ridesolo]

Nice build! I had a black '85 GPZ, what a great and fun bike; pretty state of the art back when it was new! I always wondered what the thing would look like w/ the plastic gone. All that anti-dive, stuff on the front forks probably was a real pain to work on, was it, worth the effort? I never thought the whole system was worth much on mine.

I hope you get lots of enjoyment out of it!

 

[pablohoney]

Thanks! Yeah, that anti-dive stuff was a pain, not worth the effort really, but didn't really want to hassle/pay for forks from a different bike. Didn't notice a difference in diving or handling from my 1979 xs650.
Was a fun bike, but I am kinda glad to be done with it. Sucker left me stranded most of the rides I took it on. My fault, but it wasn't cared for very well in the past 28 years.