Brake Master cylinder question.

[GS750GUY]

Gs750Guy here again.
My master cylinder on my front handle bar brake on my 78 GS750EC seems that it will not push brake fluid throuh the brake lines even after I have cleaned the master cylinder and even with the bleed nipples wide open.
I notice when I had the master cylinder all disassembled that the bore of the cylinder had corrosion pits in it. It appeared the fluid was probably the original brake fluid, was horribly dirty, and the color of mollases. I read where brake fluid will attract moisture over time and I wonder if possibly this moisture has corroded the cylider walls of my master cylinder.
If this is the problem does anyone out there have some suggestions for options. I plan on replacing all of the brake lines. Will I also need to rebuild my brake calipers?
Thanks,
GS750GUY....Harold

[Guest]

You'll need to bleed the M/C itself by pushing in the piston with the brake lever off with a phillip screwdriver until the tiny bubbles stop rising.

[duaneage]

I hate to ask this but did you use a solid bolt to attach the line instead of the proper one with the hole through it?

[Nessism]

What Chef said...you need to prime the master cylinder before it will pump fluid. The pitting you describe is not good though. Not sure what to say about that. I would strongly recommend that you take the brake caliper(s) apart and clean everything. That gunk will have gummed up the caliper as well as the master.

Good luck.

[SqDancerLynn1]

I had problem with a 77GS750 / sat for 20 years COuld Not get any fluid thru. One of the brake lines was blocked

[GS750GUY]

Master cylinder question

You'll need to bleed the M/C itself by pushing in the piston with the brake lever off with a phillip screwdriver until the tiny bubbles stop rising.

Thanks Chef,
Do I do this with the resivoir cap removed? Why with the brake lever off? does the screwdriver push the rod into the cylinder further?
GS750Guy

[GS750GUY]

I hate to ask this but did you use a solid bolt to attach the line instead of the proper one with the hole through it?

Nope, used correct bolt.

[LeeGS550E]

Lightly tap on the body of the m/c while you are pumping it. This helps knock the bubbles loose.

[unchargedken]

Double check the bleed hole in the bottom of the M/C reservoir. I going through the same thing with my 79 GS 1000s. I would consider honing the cylinder and installing new internal parts but they don't sell a rebuild kit have to buy parts separately Good luck.
ken

[Guest]

Thanks Chef,
Do I do this with the resivoir cap removed? Why with the brake lever off? does the screwdriver push the rod into the cylinder further?
GS750Guy

Yes with cap off and yes again. The lever only pushes the lever in so far and air likes to hide in that area.

[GS750GUY]

Guys, I cleaned the M/C clean as a whistle, reinstalled the M/C, opened the bleed screws, removed the brake lever, poured in fresh fluid, left the cap off and pushed in the shaft as far as it would go several times and lightly rapped on the resevoir with a screw driver. I got some bubbles but still after workin on this for an hour still could not get the cylinder to pump fluid. I know I got all the parts back in correctly.
There does not seem to be any parts that I could have reinserted backwards. It's pretty simple.
Any other suggestions on priming the cylinder? I know at least one of the brake lines is open because I can see some movement of fluid from one of the calipers in the clear rubber hose on the bleed screw when I activate the brake lever. I just can't seem to get the M/C to move fluid from the resevoir. I know all of the small passages are open.
Also, someone mentioned that parts kits are hard to find for these and parts may have to be bought seperately. Is this also true for caliper kits? Does anyone have suggestions where parts can be found? Or do I have to scrap the M/C and calipers all together and use parts from another bike?
Thanks for your suggestions.
Gs750Guy......Harold

[txironhead]

If the bore was too pitted, you're not getting enough pressure to push the fluid through.

What I'd do (and what I did on my GS) is replace the MC with a newer, one-piece MC. Mine's off of a Ninja, and I got it off of Ebay for $9 practically new and ready to use. Just make sure that the donor bike has the same number of front discs as your bike. You'll have to cut the brake switch wires and install some small connectors, but now you have a newer MC, better brake light switch, and it's easier and probably cheaper to find rebuild kits should you ever need one. Plus, I now have the added benefit of having an adjustable brake lever.

BTW, if you get a MC off of a Kawasaki, you'll need to use the Kawasaki banjo bolt for the MC. Other than that and the switch, everything else bolts right up. Make sure it's for 7/8" handlebars.

[Schweisshund]

ok ... I'll throw this out there even if I may be way off. The caliper pistons (not sure if that is what it is called) may be froze. I would take them out and lightly sand them, put on some lucas oil and reinsert. This happened to my Honda Nighthawk when I was putting it together. It worked like a charm.

[Schweisshund]

Like I said, my terminology may not be correct - but its the plunger/piston thats puts the pressure on the brake pad to release or brake. Mine were rusted into place and I could not figure it out for the longest time - I thought the m/c was not working correctly. One of my neighbors, a very old man, came over and pointed it out to me. He spent the afternoons with me helping me put the nighthawk together - we had a ball :-D

[Nessism]

Take the master back off and feed some brake fluid into the outlet fitting. You may have to play with the lever as well to find a position where you can get some fluid on the inside. Kind of hard to describe, and I don't know exactly what happened, but I had to mess around with my old 450 master this way before it would start pumping.