78 GS 750 front brake master cylinder

[Gober]

Hey everyone... I'm new to the group and hoping to learn from someone who's more knowledgeable about this topic than myself. 

 

 

 

I'm helping my 22 year old son get a 78 GS750 up and running.  We've got it running, but I'm currently battling a front brake issue.  The front brakes don't fully retract when you release the handle.  Initially, I thought it was a front caliper issue.  It's not.  Using a C clamp, I can fully compress the calipers with any union on the hydraulic hose disconnected.  As soon as it's hooked "fully up", I can no longer compress the caliper.  I'm convinced it's an issue with the master cylinder.  

 

 

 

I've disassembled it and cleaned it SEVERAL times.  No change.  Here's my question.  If you remove the cap and the plastic reservoir and you are looking at the metal base of the master cylinder, there is small hole that the fluid leaves the reservoir through and right next to it is what appears to be another smaller hole.  But on mine, that hole doesn't go into the cylinder bore.  I've looked at it through a microscope, it does not go through into the piston bore.  Should it?  If not, does anyone have a good reason why the fluid will go one direction, but not the other?  

[Dezza]

Yes, the hole should enable fluid to pass from the reservoir to the master cylinder. When the brake is applied, the piston sweeps past the hole and covers it so pressurized  fluid doesn't go back up into the reservoir. It sounds as if the hole is blocked with fossilized cack. Maybe a bit of wire will help shift it initially. Dismantling master cylinders and then shoving all the bits in an ultrasonic cleaner also often works wonders.

Also, if you have a spare known-to-be-good master cylinder you could fit that, bleed it up, and confirm the calipers are OK. (Depending on the size of the mc though, the lever feel may not be as expected.)