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Brake Master Cylinder Priming.

  • Thread starter Thread starter GS750GUY
  • Start date Start date
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GS750GUY

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GS750Guy here again.
Anyone out there have any tips for priming brake master cylinders? I just replaced my brake pads & lines and overhauled all three brake calipers on my 78GS750EC. I seem to be having trouble getting the front master cylinder to move fluid.

Someone suggested that I needed to be sure the M/C is primed with fluid before it will start moving fluid through the system.
Anyone out there have any suggestions on how to do that?

The system is as clean as a whistle and all internal parts look great.

Question: I notice in the REAR master cylinder there are two holes that pass fluid or act as a vent into the cylinder part of the cylinder tube and I can only find one hole in the bottom of the front fluid cup that allows fluid to pass into the cylinder tube. Am I possibly overlooking a tiny vent hole somewhere in the front fluid cup? Look as I may I can't seem to find more than one fluid hole.

Thanks,
GS750GUY
 
One of the two holes in the bottom of the m/c res is tiny. Poke it out with something equally tiny.

To prime, squirt fluid down the outlet and make sure fluid is in the res. Put thumb over outlet and pump cyl until you feel pressure. Hook it up to the bike and start a'bleedin'.
 
If you don't blead the master first you will get nowhere
 
instead of using your thumb, get a screw/bolt that fits in that opening and squeeze the brake lever several times. Or, do what I do - get a good spray bottle, take the sprayer out and attach it to clear tubing that will fit over that opening (may need to stretch it with needle nose pliers to get it big enough to go over the opening and seal air out) fill reservoir with fluid then pump the sprayer. I use a brake caliper bleeder bolt that will fit in the m/c opening and attach tubing to that. Makes this job a h%ll of a lot easier. Keep pumping the sprayer till all the airbubles stop coming out and you have a solid stream of brake fluid.
 
Try as I may I can only see one hole in the bottom of my M/C fluid reservoir. Am I overlooking it somewhere? How tiny is the second hole?
Do all of you other 78 GS750 guys out there have two holes in the bottom of your front brake master cylinder reservoir?
The largest one is easy to see well enough but the 2nd one is eluding me.
Thanks!
Gs750Guy
 
the 2nd, or little hole, will be between the bigger opening you see and the opening for the brake line attrachment - probably only 1/4 to 1/2" from the hole you found. It MIGHT be hiding under a little metal flap that extends over the top of the hole to keep fluid from spewing out of the cylinder when you grab the brakes. This second hole is tiny, barely big enough to fit the metal wire from a bread wrapper twist tie into it for cleaning purposes.
 
You have a 78 so probably the round master cyl? I'm not familiar with them but I'm sure they must operate similarly hence the second hole. One hole is intake and one is exhaust -- kind of like a two-cycle motor I think.
 
Brake system bleeding question

Brake system bleeding question

I have completely replaced all front brake lines on my 78 GS750EC, rebuilt the calipers and master cylinder. The system is completely dry. I have I think primed the M/C and have followed my service manuals instruction for bleeding the brakes. I have gone through the bleeding cycle a hundred times and have yet to get a drop of new brake fluid into the system. I have removed the brake line at the M/C and have tested it for vacuum pressure against my thumb and force pressure into my thumb and it seems to move fluid slightly when the lever is squeezed. I can't seem to figure out what I'm doing wrong. Should the M/C by itself move enough fluid to charge a completely dry system very quickly? Do I need to cycle the M/C a couple of hundred times to charge the system since it is dry. Or do I need the help of some kind of vacuum pump or something to help draw in new fluid from the caliper end? Do I possibly have a bad M/C? There was some minor corrosion in the M/C bore but I would figure that since the M/C has good suction as well as pressure that it must be o.k.Someone told me that using a vacuum at the caliper end and drawing fluid from the M/C is another way to charge the system but I'm not sure how to do that. And how would vacuum draw fluid out of the reservoir and past the plunger in the M/C? HELP!!!!!! I'M SO CLOSE TO RIDING THIS THING AND RUNNING OUT OF SEASON.GS750GUY
 
When I rebuilt my master cylinder, I put a spring in first instead of the seal...and it would never build pressure. I know it's a biatch to remove the c-clip to look at how you put the MC back together, but you really should confirm you assembled everything properly.

I spent a whole bottle of fluid on my bike before I found out the MC wasn't working properly.

After you confirm the MC assembled right, I usually use a vacuum pump to pull fluid to the calipers. If you don't have that, I've had to disconnect the banjo at the calipers and pump the MC a few times to get the lines filled and to start to build up some pressure.

Eventually when you think you have it pretty good...use a tie strap to pull the brake lever back to the throttle, loosen the cap, and leave it sit overnight. This will also help move some air up out of the system naturally. You can tap the MC slightly with a hammer too to accelerate this.

~Adam
 
It takes a long time and lots of patience to get anywhere with a completely dry system. When I did my m/c I primed it, mounted it, filled it up, and spent the next hour jiggling the handle and watching bubbles float up. It wasn't until much later that it built any kind of pressure. Eventually with the jiggling and wiggling and banging (w/ rubber mallet) I got most of the air out of the m/c, then went on to bleed the rest of the system normally.

Don't waste your time bleeding at the calipers until the m/c stops bubbling.

Are you seeing any bubbles rise when you jiggle the handle (press in like 1/4 way, then let out repeatedly)??? If so, you're on the right track -- keep at it.
 
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