Master Cylinder

[Dim Pete]

Hi, got my first classic truck a couple months ago, 1972 GMC 1500. Last week the dash brake light came on, I checked the fluid and the front level was a little low. If the master cylinder was leaking into the booster, would both the front and back sections of the cylinder lose fluid? I do not see any obvious leaks around the brakes, and they work fine. I do not want to replace a master cylinder if that is not the problem. Also, how do I reset the dash brake light. Thanks for the help.

[davepl]

I've never seen one leak back into the booster, though I suppose anything is possible.

If the dash light is staying on then one side of the system is effectively EMPTY (on the line side, not the reservoir side), or at least cannot build pressure, and the piston in between them has slid all the way to one end. I think you may have to remove the center sender and stick something down in the hole to re-center it, but hold off until someone confirms that, because I've never had it happen to me.