I'm loosing brake fluid from the front half of the master cylinder. If I'm not mistaken the front half controls the rear brakes. It will drop all the way to the bottom in a few months letting air in if I dont keep an eye on it. The rear half of the master cylinder stays completely full. I don't see any evidence of a leak anywhere. The outside of the master cylinder sometimes seems to be wet as if it's over flowing. The fact that the rear half stays completely full has me wondering is somehow the fluid is being forced from the front half to the rear half of the master cylinder.
I had to bleed the brakes a few months ago when this happened, after bleeding, the brakes are fine, the pedals firm, but it will still leak down eventually.
Has anyone else had the problem?

 

The master cylinder ma have a bad o-ring on the piston. Or you may have a leak in the lines that are left front and right rear or right front and left rear.

 

I don' think the 93's are diagonally balanced, they only have antilock on the rear wheels.

 

Sounds like the master has gone bad. Look inside your truck behind the floor matting also for fluid and Ive seen the boosters get fluid in them.

 

look for line leakage along the frame rail behind the fuel tank. also look close at the rear backing plate area.

 

93w250

I have the exact same problem, I have been looking for 11/2 years for the leak with no success. good luck.

 

I've been away for a week, thanks for the replies.
I've checked the lines, bleed screws, etc and see no sign of a leak. I'll check up under the dash, let you all know what if anything I find.

 

Well I finally had a chance to examine the brake system again and here's what I found out. I still see not evidence of an external leak. I removed the master cylinder cover and pushed in the pedal and brake fluid squirted out of the front half of the resevoir. There must be an internal leak allowing pressure to back up into it. As I stated in my original post, the outside of the master cyl. always appears to be wet. The pressure must be forcing the fluid past the cover gasket. I went to Advanced, got a new Bendix master cyl. and front pads.

 

Fluid squirting up is normal as long as it stops after 1/2 in of pedal travel. When the pedal is released, the piston uncovers the fluid return orifice to releive any excess pressure or fluid or add fluid due to expansion/contraction or wear. When the piston starts to travel the fluid releives upward thru the return orifice until it goes past it then builds pressure to apply the brakes. If the master is wet with fluid, try cleaning the rubber diaphram gasket and top of cyl. If diaphram is extended out, reseat the diaphram before installing the cover so you don't displace the fluid when installing the cover. If your reservoir is going dry, you have a leak in the hydraulic system somewhere. Follow all lines and hoses and look for a wet spot. Get a buddy to stand on the pedal while you go under the truck to inspect it. If you don't find anything there, start pulling wheels to inspect calipers and wheel cyls. If the cyls look dry, pop the boot to check for fluid inside the boot. Good luck