The GMC CCKW

I have a gmc 352,i have just fitted a rebuilt master cylinder, along with rebuilt,hydrovac and wheel cylinders. I have bled the whole sytsem and set up the brakes, i have driventhe trucks and brakes seem to work fine. The problem i have is that when you enitially press the brake pedal there the brake fluid is coming out of the breather holes in the cap of the master cylinder. the level is about 1inch down from the top. i took the cap off the master cylinder and pushed the brake pedal very slowly and there is a small jet of brake fluid created is this normal..

Thanks

Howard

The answer to your question is both yes and no. Yes, a small jet of fluid may be squirted back up into the master cylinder reservoir through the compensating port on brake application but this should not be powerful enough to appear outside the master cylinder. The compensating port is a tiny hole about 1/16" diameter that is just forward of the piston cup when the piston is in the brakes released position.
If you look into the reservoir you will notice two holes that go through into the working part of the cylinder. Both appear to be around 3/16" diameter. Don't be fooled, the rear one is full diameter right through whilst the forward one reduces to the very small diameter hole I mentioned. If the cylinder has been re-sleeved it is possible that this tiny hole has been incorrectly drilled.
Another possibility is that the piston/cup is going too far back in the off position. This could be caused by incorrect piston, incorrect cup or omission or installation of too thin a washer adjacent the piston retainer cir clip.

David