Topic: Frickin' master cylinder. No pressure / loses pressure

[greenjeans]

Help me pin point the most likely problem.  New master cylinder rebuild.  New caliper seal as well.  Bottom lines are new.  Top line is used, but good shape.  All parts thoroughly cleaned prior to reassembly.  I am getting next to no lever or pressure.  I left the lever tied over night which usually works.  I've bled quite a bit of fluid through the system.  It barely builds enough pressure to move the piston and activate the pressure switch for the rear brake light.  As soon as you bottom out the lever, the brake light will flash and you can see the caliper move, but all pressure is gone in a second or less.  There are no leaks - dry as a bone from all parts.

Do I still have a huge air bubble?    How would it lose any pressure without leaking?
The bore of the mc looked great.  The parts are in the correct order.  I used the piston original to the mc bore but replaced the rubber parts.  Thoroughly cleaned.

I've only rebuilt a dozen or more of these over the years. I'm stumped.

[Don R]

 The usual answer is the small air vent hole in the M/C is plugged. I've had some really stubborn ones too. One guy recently used a squeeze bottle to move fluid up from the caliper into the M/C. taking the air it's natural direction too. Sometimes if you pull the lever all the way back to the grip fluid will pass inside the m/c and leak out from the seal near the pivot bolt. Watch for that too, it can drip on the tank.

[pjlogue]

The usual answer is the small air vent hole in the M/C is plugged. I've had some really stubborn ones too. One guy recently used a squeeze bottle to move fluid up from the caliper into the M/C. taking the air it's natural direction too. Sometimes if you pull the lever all the way back to the grip fluid will pass inside the m/c and leak out from the seal near the pivot bolt. Watch for that too, it can drip on the tank.

What DonR said!  If you pump the brake lever quickly and you don't build pressure and you don't have leaks then it points to an air bubble in the MC.  If you pump quickly and do build pressure then the air is in the line or the caliper. 

I have seen many posts where people tie their brake lever to the grip.  Honda service manual says do not do this.  Use a 1 cm block to keep the lever out a bit to prevent what DonR points out. 

-P.

[greenjeans]

Both of the holes are clear.  1 will shot fluid well out of the reservoir.  The other, you can see tiny bubbles or fluid moving. 
I'll give it another shot this evening.  I double-checked this morning.  Everything still dry.  Maybe a day off will cure it.

[greenjeans]

Definitely going to try a small block to keep the lever off the grip. 

Should both holes be the same size?  Seems like one definitely moves more fluid than the other.

And do they both go straight into the cylinder?

[ekpent]

 One is much smaller. I use a single strand of wire from a wire brush to clean them it is that small. I have rebuilt quite a few 750 masters also and just did one very recently for a 750 F0. Keep this our secret and don't tell anybody but I had a brain fart while doing it an did not orientate the spring and a rubber bit correctly and got no love. Went back in and redid it proper and got fluid going immediately on a totally dry system.

[greenjeans]

I'm almost to the point of taking it back apart and double-checking the orientation of things even though I'm almost positive they are in there in the correct order.
We shall see.

[ekpent]

 I'll testify that they come apart a lot easier the second time with nice new parts !

[Don R]

One is much smaller. I use a single strand of wire from a wire brush to clean them it is that small. I have rebuilt quite a few 750 masters also and just did one very recently for a 750 F0. Keep this our secret and don't tell anybody but I had a brain fart while doing it an did not orientate the spring and a rubber bit correctly and got no love. Went back in and redid it proper and got fluid going immediately on a totally dry system.

 Me too, shh.